# Copyright (c) 2021 PaddlePaddle Authors. All Rights Reserved.
# Copyright 2021 Microsoft Research and The HuggingFace Inc. team. All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

import math
import os
from abc import abstractmethod
from collections import namedtuple

import numpy as np
import paddle
import paddle.nn as nn
import paddle.nn.functional as F
from paddle import ParamAttr
from paddle.nn import Layer
from paddle.utils import try_import


def read_config(fp=None):
    if fp is None:
        dir_name = os.path.dirname(os.path.abspath(__file__))
        fp = os.path.join(dir_name, "visual_backbone.yaml")
    with open(fp, "r") as fin:
        yacs_config = try_import("yacs.config")
        cfg = yacs_config.CfgNode().load_cfg(fin)
    cfg.freeze()
    return cfg


class Conv2d(nn.Conv2D):
    def __init__(self, *args, **kwargs):
        norm = kwargs.pop("norm", None)
        activation = kwargs.pop("activation", None)
        super(Conv2d, self).__init__(*args, **kwargs)

        self.norm = norm
        self.activation = activation

    def forward(self, x):
        x = super(Conv2d, self).forward(x)
        if self.norm is not None:
            x = self.norm(x)
        if self.activation is not None:
            x = self.activation(x)
        return x


class CNNBlockBase(Layer):
    def __init__(self, in_channels, out_channels, stride):
        """
        The `__init__` method of any subclass should also contain these arguments.
        Args:
            in_channels (int):
            out_channels (int):
            stride (int):
        """
        super(CNNBlockBase, self).__init__()
        self.in_channels = in_channels
        self.out_channels = out_channels
        self.stride = stride

    def freeze(self):
        for p in self.parameters():
            p.stop_gradient = True


ResNetBlockBase = CNNBlockBase


class ShapeSpec(namedtuple("_ShapeSpec", ["channels", "height", "width", "stride"])):
    def __new__(cls, channels=None, height=None, width=None, stride=None):
        return super().__new__(cls, channels, height, width, stride)


def get_norm(norm, out_channels):
    """
    Args:
        norm (str or callable): either one of BN, SyncBN, FrozenBN, GN;
            or a callable that takes a channel number and returns
            the normalization layer as a nn.Layer.
        out_channels (int): out_channels
    Returns:
        nn.Layer or None: the normalization layer
    """
    if norm is None:
        return None
    if isinstance(norm, str):
        if len(norm) == 0:
            return None
        norm = {
            "BN": nn.BatchNorm,
            "SyncBN": nn.SyncBatchNorm,
            "FrozenBN": FrozenBatchNorm,
        }[norm]
    return norm(out_channels)


class FrozenBatchNorm(nn.BatchNorm):
    def __init__(self, num_channels):
        param_attr = ParamAttr(learning_rate=0.0, trainable=False)
        bias_attr = ParamAttr(learning_rate=0.0, trainable=False)
        super(FrozenBatchNorm, self).__init__(
            num_channels, param_attr=param_attr, bias_attr=bias_attr, use_global_stats=True
        )


class Backbone(nn.Layer):
    def __init__(self):
        super(Backbone, self).__init__()

    @abstractmethod
    def forward(self, *args):
        pass

    @property
    def size_divisibility(self) -> int:
        return 0

    def output_shape(self):
        # this is a backward-compatible default
        return {
            name: ShapeSpec(channels=self._out_feature_channels[name], stride=self._out_feature_strides[name])
            for name in self._out_features
        }


class BasicBlock(CNNBlockBase):
    """
    The basic residual block for ResNet-18 and ResNet-34 defined in :paper:`ResNet`,
    with two 3x3 conv layers and a projection shortcut if needed.
    """

    def __init__(self, in_channels, out_channels, *, stride=1, norm="BN"):
        raise NotImplementedError


class BottleneckBlock(CNNBlockBase):
    """
    The standard bottleneck residual block used by ResNet-50, 101 and 152
    defined in :paper:`ResNet`.  It contains 3 conv layers with kernels
    1x1, 3x3, 1x1, and a projection shortcut if needed.
    """

    def __init__(
        self,
        in_channels,
        out_channels,
        *,
        bottleneck_channels,
        stride=1,
        num_groups=1,
        norm="BN",
        stride_in_1x1=False,
        dilation=1,
    ):
        super(BottleneckBlock, self).__init__(in_channels, out_channels, stride)

        if in_channels != out_channels:
            self.shortcut = Conv2d(
                in_channels,
                out_channels,
                kernel_size=1,
                stride=stride,
                bias_attr=False,
                norm=get_norm(norm, out_channels),
            )
        else:
            self.shortcut = None

        stride_1x1, stride_3x3 = (stride, 1) if stride_in_1x1 else (1, stride)

        self.conv1 = Conv2d(
            in_channels,
            bottleneck_channels,
            kernel_size=1,
            stride=stride_1x1,
            bias_attr=False,
            norm=get_norm(norm, bottleneck_channels),
        )

        self.conv2 = Conv2d(
            bottleneck_channels,
            bottleneck_channels,
            kernel_size=3,
            stride=stride_3x3,
            padding=1 * dilation,
            bias_attr=False,
            groups=num_groups,
            dilation=dilation,
            norm=get_norm(norm, bottleneck_channels),
        )

        self.conv3 = Conv2d(
            bottleneck_channels,
            out_channels,
            kernel_size=1,
            bias_attr=False,
            norm=get_norm(norm, out_channels),
        )
        # init code is removed cause pretrained model will be loaded

    def forward(self, x):
        out = self.conv1(x)
        out = F.relu(out)

        out = self.conv2(out)
        out = F.relu(out)

        out = self.conv3(out)

        if self.shortcut is not None:
            shortcut = self.shortcut(x)
        else:
            shortcut = x

        out += shortcut
        out = F.relu(out)
        return out


class DeformBottleneckBlock(CNNBlockBase):
    """
    Similar to :class:`BottleneckBlock`, but with :paper:`deformable conv <deformconv>`
    in the 3x3 convolution.
    """

    def __init__(
        self,
        in_channels,
        out_channels,
        *,
        bottleneck_channels,
        stride=1,
        num_groups=1,
        norm="BN",
        stride_in_1x1=False,
        dilation=1,
        deform_modulated=False,
        deform_num_groups=1,
    ):
        raise NotImplementedError


class BasicStem(CNNBlockBase):
    """
    The standard ResNet stem (layers before the first residual block),
    with a conv, relu and max_pool.
    """

    def __init__(self, in_channels=3, out_channels=64, norm="BN"):
        """
        Args:
            norm (str or callable): norm after the first conv layer.
                See :func:`layers.get_norm` for supported format.
        """
        super(BasicStem, self).__init__(in_channels, out_channels, 4)
        self.in_channels = in_channels
        self.conv1 = Conv2d(
            in_channels,
            out_channels,
            kernel_size=7,
            stride=2,
            padding=3,
            bias_attr=False,
            norm=get_norm(norm, out_channels),
        )

    def forward(self, x):
        x = self.conv1(x)
        x = F.relu(x)
        x = F.max_pool2d(x, kernel_size=3, stride=2, padding=1)
        return x


class ResNet(Backbone):
    def __init__(self, stem, stages, num_classes=None, out_features=None, freeze_at=0):
        super(ResNet, self).__init__()
        self.stem = stem
        self.num_classes = num_classes

        current_stride = self.stem.stride
        self._out_feature_strides = {"stem": current_stride}
        self._out_feature_channels = {"stem": self.stem.out_channels}

        self.stage_names, self.stages = [], []

        if out_features is not None:
            num_stages = max([{"res2": 1, "res3": 2, "res4": 3, "res5": 4}.get(f, 0) for f in out_features])
            stages = stages[:num_stages]
        for i, blocks in enumerate(stages):
            assert len(blocks) > 0, len(blocks)
            for block in blocks:
                assert isinstance(block, CNNBlockBase), block

            name = "res" + str(i + 2)
            stage = nn.Sequential(*blocks)

            self.add_sublayer(name, stage)
            self.stage_names.append(name)
            self.stages.append(stage)

            self._out_feature_strides[name] = current_stride = int(
                current_stride * np.prod([k.stride for k in blocks])
            )
            self._out_feature_channels[name] = curr_channels = blocks[-1].out_channels
        self.stage_names = tuple(self.stage_names)

        if num_classes is not None:
            self.avgpool = nn.AdaptiveAvgPool2D(1)
            self.linear = nn.Linear(curr_channels, num_classes)
            name = "linear"

        if out_features is None:
            out_features = [name]
        self._out_features = out_features
        assert len(self._out_features)
        children = [x[0] for x in self.named_children()]
        for out_feature in self._out_features:
            assert out_feature in children, "Available children: {}".format(", ".join(children))
        self.freeze(freeze_at)

    def forward(self, x):
        """
        Args:
            x: Tensor of shape (N,C,H,W). H, W must be a multiple of ``self.size_divisibility``.

        Returns:
            dict[str->Tensor]: names and the corresponding features
        """
        assert x.dim() == 4, f"ResNet takes an input of shape (N, C, H, W). Got {x.shape} instead!"
        outputs = {}
        x = self.stem(x)
        if "stem" in self._out_features:
            outputs["stem"] = x
        for name, stage in zip(self.stage_names, self.stages):
            x = stage(x)
            if name in self._out_features:
                outputs[name] = x
        if self.num_classes is not None:
            x = self.avgpool(x)
            x = paddle.flatten(x, 1)
            x = self.linear(x)
            if "linear" in self._out_features:
                outputs["linear"] = x
        return outputs

    def output_shape(self):
        return {
            name: ShapeSpec(channels=self._out_feature_channels[name], stride=self._out_feature_strides[name])
            for name in self._out_features
        }

    @staticmethod
    def make_stage(block_class, num_blocks, *, in_channels, out_channels, **kwargs):
        """
        Create a list of blocks of the same type that forms one ResNet stage.

        Args:
            block_class (type): a subclass of CNNBlockBase that's used to create all blocks in this
                stage. A module of this type must not change spatial resolution of inputs unless its
                stride != 1.
            num_blocks (int): number of blocks in this stage
            in_channels (int): input channels of the entire stage.
            out_channels (int): output channels of **every block** in the stage.
            kwargs: other arguments passed to the constructor of
                `block_class`. If the argument name is "xx_per_block", the
                argument is a list of values to be passed to each block in the
                stage. Otherwise, the same argument is passed to every block
                in the stage.

        Returns:
            list[CNNBlockBase]: a list of block module.

        Examples:
        ::
            stage = ResNet.make_stage(
                BottleneckBlock, 3, in_channels=16, out_channels=64,
                bottleneck_channels=16, num_groups=1,
                stride_per_block=[2, 1, 1],
                dilations_per_block=[1, 1, 2]
            )

        Usually, layers that produce the same feature map spatial size are defined as one
        "stage" (in :paper:`FPN`). Under such definition, ``stride_per_block[1:]`` should
        all be 1.
        """
        blocks = []
        for i in range(num_blocks):
            curr_kwargs = {}
            for k, v in kwargs.items():
                if k.endswith("_per_block"):
                    assert len(v) == num_blocks, (
                        f"Argument '{k}' of make_stage should have the " f"same length as num_blocks={num_blocks}."
                    )
                    newk = k[: -len("_per_block")]
                    assert newk not in kwargs, f"Cannot call make_stage with both {k} and {newk}!"
                    curr_kwargs[newk] = v[i]
                else:
                    curr_kwargs[k] = v

            blocks.append(block_class(in_channels=in_channels, out_channels=out_channels, **curr_kwargs))
            in_channels = out_channels
        return blocks

    @staticmethod
    def make_default_stages(depth, block_class=None, **kwargs):
        """
        Created list of ResNet stages from pre-defined depth (one of 18, 34, 50, 101, 152).
        If it doesn't create the ResNet variant you need, please use :meth:`make_stage`
        instead for fine-grained customization.

        Args:
            depth (int): depth of ResNet
            block_class (type): the CNN block class. Has to accept
                `bottleneck_channels` argument for depth > 50.
                By default it is BasicBlock or BottleneckBlock, based on the
                depth.
            kwargs:
                other arguments to pass to `make_stage`. Should not contain
                stride and channels, as they are predefined for each depth.

        Returns:
            list[list[CNNBlockBase]]: modules in all stages; see arguments of
                :class:`ResNet.__init__`.
        """
        num_blocks_per_stage = {
            18: [2, 2, 2, 2],
            34: [3, 4, 6, 3],
            50: [3, 4, 6, 3],
            101: [3, 4, 23, 3],
            152: [3, 8, 36, 3],
        }[depth]
        if block_class is None:
            block_class = BasicBlock if depth < 50 else BottleneckBlock
        if depth < 50:
            in_channels = [64, 64, 128, 256]
            out_channels = [64, 128, 256, 512]
        else:
            in_channels = [64, 256, 512, 1024]
            out_channels = [256, 512, 1024, 2048]
        ret = []
        for (n, s, i, o) in zip(num_blocks_per_stage, [1, 2, 2, 2], in_channels, out_channels):
            if depth >= 50:
                kwargs["bottleneck_channels"] = o // 4
            ret.append(
                ResNet.make_stage(
                    block_class=block_class,
                    num_blocks=n,
                    stride_per_block=[s] + [1] * (n - 1),
                    in_channels=i,
                    out_channels=o,
                    **kwargs,
                )
            )
        return ret

    def freeze(self, freeze_at=0):
        if freeze_at >= 1:
            self.stem.freeze()
        for idx, stage in enumerate(self.stages, start=2):
            if freeze_at >= idx:
                for block in stage.children():
                    block.freeze()
        return self


class LastLevelMaxPool(nn.Layer):
    """
    This module is used in the original FPN to generate a downsampled
    P6 feature from P5.
    """

    def __init__(self):
        super(LastLevelMaxPool, self).__init__()
        self.num_levels = 1
        self.in_feature = "p5"

    def forward(self, x):
        return [F.max_pool2d(x, kernel_size=1, stride=2, padding=0)]


def _assert_strides_are_log2_contiguous(strides):
    """
    Assert that each stride is 2x times its preceding stride, i.e. "contiguous in log2".
    """
    for i, stride in enumerate(strides[1:], 1):
        assert stride == 2 * strides[i - 1], "Strides {} {} are not log2 contiguous".format(stride, strides[i - 1])


class FPN(Backbone):
    def __init__(self, bottom_up, in_features, out_channels, norm="", top_block=None, fuse_type="sum"):
        super(FPN, self).__init__()
        assert isinstance(bottom_up, Backbone)
        assert in_features, in_features

        # Feature map strides and channels from the bottom up network (e.g. ResNet)
        input_shapes = bottom_up.output_shape()
        strides = [input_shapes[f].stride for f in in_features]
        in_channels_per_feature = [input_shapes[f].channels for f in in_features]

        _assert_strides_are_log2_contiguous(strides)
        lateral_convs = []
        output_convs = []

        use_bias = norm == ""
        for idx, in_channels in enumerate(in_channels_per_feature):
            lateral_norm = get_norm(norm, out_channels)
            output_norm = get_norm(norm, out_channels)

            lateral_conv = Conv2d(in_channels, out_channels, kernel_size=1, bias_attr=use_bias, norm=lateral_norm)
            output_conv = Conv2d(
                out_channels,
                out_channels,
                kernel_size=3,
                stride=1,
                padding=1,
                bias_attr=use_bias,
                norm=output_norm,
            )
            stage = int(math.log2(strides[idx]))
            self.add_sublayer("fpn_lateral{}".format(stage), lateral_conv)
            self.add_sublayer("fpn_output{}".format(stage), output_conv)

            lateral_convs.append(lateral_conv)
            output_convs.append(output_conv)
        # Place convs into top-down order (from low to high resolution)
        # to make the top-down computation in forward clearer.
        self.lateral_convs = lateral_convs[::-1]
        self.output_convs = output_convs[::-1]
        self.top_block = top_block
        self.in_features = tuple(in_features)
        self.bottom_up = bottom_up
        # Return feature names are "p<stage>", like ["p2", "p3", ..., "p6"]
        self._out_feature_strides = {"p{}".format(int(math.log2(s))): s for s in strides}
        # top block output feature maps.
        if self.top_block is not None:
            for s in range(stage, stage + self.top_block.num_levels):
                self._out_feature_strides["p{}".format(s + 1)] = 2 ** (s + 1)

        self._out_features = list(self._out_feature_strides.keys())
        self._out_feature_channels = {k: out_channels for k in self._out_features}
        self._size_divisibility = strides[-1]
        assert fuse_type in {"avg", "sum"}
        self._fuse_type = fuse_type

    @property
    def size_divisibility(self):
        return self._size_divisibility

    def forward(self, x):
        """
        Args:
            x (dict[str->Tensor]): mapping feature map name (e.g., "res5") to
                feature map tensor for each feature level in high to low resolution order.

        Returns:
            dict[str->Tensor]:
                mapping from feature map name to FPN feature map tensor
                in high to low resolution order. Returned feature names follow the FPN
                paper convention: "p<stage>", where stage has stride = 2 ** stage e.g.,
                ["p2", "p3", ..., "p6"].
        """
        bottom_up_features = self.bottom_up(x)
        results = []
        prev_features = self.lateral_convs[0](bottom_up_features[self.in_features[-1]])
        results.append(self.output_convs[0](prev_features))

        # Reverse feature maps into top-down order (from low to high resolution)
        for idx, (lateral_conv, output_conv) in enumerate(zip(self.lateral_convs, self.output_convs)):
            if idx > 0:
                features = self.in_features[-idx - 1]
                features = bottom_up_features[features]
                top_down_features = F.interpolate(prev_features, scale_factor=2.0, mode="nearest")
                lateral_features = lateral_conv(features)
                prev_features = lateral_features + top_down_features
                if self._fuse_type == "avg":
                    prev_features /= 2
                results.insert(0, output_conv(prev_features))

        if self.top_block is not None:
            if self.top_block.in_feature in bottom_up_features:
                top_block_in_feature = bottom_up_features[self.top_block.in_feature]
            else:
                top_block_in_feature = results[self._out_features.index(self.top_block.in_feature)]
            results.extend(self.top_block(top_block_in_feature))
        assert len(self._out_features) == len(results)
        return {f: res for f, res in zip(self._out_features, results)}

    def output_shape(self):
        return {
            name: ShapeSpec(channels=self._out_feature_channels[name], stride=self._out_feature_strides[name])
            for name in self._out_features
        }


def make_stage(*args, **kwargs):
    """
    Deprecated alias for backward compatibility.
    """
    return ResNet.make_stage(*args, **kwargs)


def build_resnet_backbone(cfg, input_shape=None):
    """
    Create a ResNet instance from config.

    Returns:
        ResNet: a :class:`ResNet` instance.
    """
    # need registration of new blocks/stems?
    if input_shape is None:
        ch = 3
    else:
        ch = input_shape.channels
    norm = cfg.MODEL.RESNETS.NORM
    stem = BasicStem(
        in_channels=ch,
        out_channels=cfg.MODEL.RESNETS.STEM_OUT_CHANNELS,
        norm=norm,
    )

    # fmt: off
    freeze_at = cfg.MODEL.BACKBONE.FREEZE_AT  # default as 2
    out_features = cfg.MODEL.RESNETS.OUT_FEATURES
    depth = cfg.MODEL.RESNETS.DEPTH
    num_groups = cfg.MODEL.RESNETS.NUM_GROUPS
    width_per_group = cfg.MODEL.RESNETS.WIDTH_PER_GROUP
    bottleneck_channels = num_groups * width_per_group
    in_channels = cfg.MODEL.RESNETS.STEM_OUT_CHANNELS
    out_channels = cfg.MODEL.RESNETS.RES2_OUT_CHANNELS
    stride_in_1x1 = cfg.MODEL.RESNETS.STRIDE_IN_1X1
    res5_dilation = cfg.MODEL.RESNETS.RES5_DILATION
    deform_on_per_stage = cfg.MODEL.RESNETS.DEFORM_ON_PER_STAGE
    deform_modulated = cfg.MODEL.RESNETS.DEFORM_MODULATED
    deform_num_groups = cfg.MODEL.RESNETS.DEFORM_NUM_GROUPS
    # fmt: on
    assert res5_dilation in {1, 2}, "res5_dilation cannot be {}.".format(res5_dilation)

    num_blocks_per_stage = {
        18: [2, 2, 2, 2],
        34: [3, 4, 6, 3],
        50: [3, 4, 6, 3],
        101: [3, 4, 23, 3],
        152: [3, 8, 36, 3],
    }[depth]

    if depth in [18, 34]:
        assert out_channels == 64, "Must set MODEL.RESNETS.RES2_OUT_CHANNELS = 64 for R18/R34"
        assert not any(deform_on_per_stage), "MODEL.RESNETS.DEFORM_ON_PER_STAGE unsupported for R18/R34"
        assert res5_dilation == 1, "Must set MODEL.RESNETS.RES5_DILATION = 1 for R18/R34"
        assert num_groups == 1, "Must set MODEL.RESNETS.NUM_GROUPS = 1 for R18/R34"

    stages = []

    for idx, stage_idx in enumerate(range(2, 6)):
        # res5_dilation is used this way as a convention in R-FCN & Deformable Conv paper
        dilation = res5_dilation if stage_idx == 5 else 1
        first_stride = 1 if idx == 0 or (stage_idx == 5 and dilation == 2) else 2
        stage_kargs = {
            "num_blocks": num_blocks_per_stage[idx],
            "stride_per_block": [first_stride] + [1] * (num_blocks_per_stage[idx] - 1),
            "in_channels": in_channels,
            "out_channels": out_channels,
            "norm": norm,
        }
        # Use BasicBlock for R18 and R34.
        if depth in [18, 34]:
            stage_kargs["block_class"] = BasicBlock
        else:
            stage_kargs["bottleneck_channels"] = bottleneck_channels
            stage_kargs["stride_in_1x1"] = stride_in_1x1
            stage_kargs["dilation"] = dilation
            stage_kargs["num_groups"] = num_groups
            if deform_on_per_stage[idx]:
                stage_kargs["block_class"] = DeformBottleneckBlock
                stage_kargs["deform_modulated"] = deform_modulated
                stage_kargs["deform_num_groups"] = deform_num_groups
            else:
                stage_kargs["block_class"] = BottleneckBlock
        blocks = ResNet.make_stage(**stage_kargs)
        in_channels = out_channels
        out_channels *= 2
        bottleneck_channels *= 2
        stages.append(blocks)
    return ResNet(stem, stages, out_features=out_features, freeze_at=freeze_at)


def build_resnet_fpn_backbone(cfg, input_shape=None):
    bottom_up = build_resnet_backbone(cfg, input_shape)
    in_features = cfg.MODEL.FPN.IN_FEATURES
    out_channels = cfg.MODEL.FPN.OUT_CHANNELS
    backbone = FPN(
        bottom_up=bottom_up,
        in_features=in_features,
        out_channels=out_channels,
        norm=cfg.MODEL.FPN.NORM,
        top_block=LastLevelMaxPool(),
        fuse_type=cfg.MODEL.FPN.FUSE_TYPE,
    )
    return backbone


class VisualBackbone(Layer):
    def __init__(self, config):
        super(VisualBackbone, self).__init__()
        self.cfg = read_config()
        self.backbone = build_resnet_fpn_backbone(self.cfg)
        # syncbn is removed cause that will cause import of torch

        assert len(self.cfg.MODEL.PIXEL_MEAN) == len(self.cfg.MODEL.PIXEL_STD)
        num_channels = len(self.cfg.MODEL.PIXEL_MEAN)
        self.register_buffer("pixel_mean", paddle.to_tensor(self.cfg.MODEL.PIXEL_MEAN).reshape([num_channels, 1, 1]))
        self.register_buffer("pixel_std", paddle.to_tensor(self.cfg.MODEL.PIXEL_STD).reshape([num_channels, 1, 1]))
        self.out_feature_key = "p2"
        # is_deterministic is disabled here.
        self.pool = nn.AdaptiveAvgPool2D(config["image_feature_pool_shape"][:2])
        if len(config["image_feature_pool_shape"]) == 2:
            config["image_feature_pool_shape"].append(self.backbone.output_shape()[self.out_feature_key].channels)
        assert self.backbone.output_shape()[self.out_feature_key].channels == config["image_feature_pool_shape"][2]

    def forward(self, images):
        images_input = (paddle.to_tensor(images) - self.pixel_mean) / self.pixel_std
        features = self.backbone(images_input)
        features = features[self.out_feature_key]
        features = self.pool(features).flatten(start_axis=2).transpose([0, 2, 1])
        return features
