[Bug] Fixing `test_gpu_sampling_DataLoader` that fails with torch 2.7.

## Description <!-- Brief description. Refer to the related issues if existed. It'll be great if relevant reviewers can be assigned as well.--> Beginning with PyTorch `2.7.0a`, the `test_gpu_sampling_DataLoader` test consistently fails, producing the following error: ``` group = group or _get_default_group() > work = group.alltoall_base( output, input, output_split_sizes, input_split_sizes, opts ) E RuntimeError: No backend type associated with device type cpu E This exception is thrown by __iter__ of MiniBatchTransformer(datapipe=Bufferer, transformer=_seeds_cooperative_exchange_1_wait_future) /usr/local/lib/python3.12/dist-packages/torch/distributed/distributed_c10d.py:4390: RuntimeError ``` Since the `gloo` backend does not support `alltoall`: ``` group = group or _get_default_group() > work = group.alltoall(output_tensor_list, input_tensor_list, opts) E RuntimeError: Backend gloo does not support alltoall E This exception is thrown by __iter__ of MiniBatchTransformer(datapipe=Bufferer, transformer=_seeds_cooperative_exchange_2) ``` and the `ucc` backend is slated for deprecation, the `mpi` backend is now the only viable option for this test. This PR introduces backward compatibility, allowing the test to run on older and newer PyTorch versions. ## Checklist Please feel free to remove inapplicable items for your PR. - [x] The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]]) - [x] I've leverage the [tools](https://docs.google.com/document/d/1iHyj7zlmygKSk5gBPsqIqL5ASPzJSPREaNT_QdsiYA4/edit) to beautify the python and c++ code. - [x] The PR is complete and small, read the [Google eng practice (CL equals to PR)](https://google.github.io/eng-practices/review/developer/small-cls.html) to understand more about small PR. In DGL, we consider PRs with less than 200 lines of core code change are small (example, test and documentation could be exempted). - [x] All changes have test coverage - [x] Code is well-documented - [x] To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change ## Changes <!-- You could use following template - [ ] Feature1, tests, (and when applicable, API doc) - [ ] Feature2, tests, (and when applicable, API doc) -->