[Bug] Fixing test_gpu_sampling_DataLoader that fails with torch 2.7.

[drivanov]

Description

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

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• 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