Fix: Attach _linear_extra_repr to quantized linear layers

I've made changes to resolve the AttributeError: 'Linear' object has no attribute '_linear_extra_repr' during quantization by adding necessary imports—import types and from torchao.quantization.quant_api import _linear_extra_repr—to torchchat/utils/quantize.py, enabling dynamic method attachment and access to _linear_extra_repr. Additionally, I modified the quantize_model function by introducing a recursive helper function _attach_extra_repr within the same file. This function traverses all modules in the model and, for each nn.Linear module missing the extra_repr attribute, dynamically attaches _linear_extra_repr using types.MethodType. The helper is invoked at the beginning of quantize_model to ensure all nn.Linear modules have the extra_repr method before quantization begins.