[train] Update Torch default timeout_s to use Torch's default timeout

[matthewdeng]

Why are these changes needed?

Previously the default was hardcoded to 30 minutes. By defaulting to None, it will use Torch Distributed's default value. For NCCL, this will be 10 minutes (see torch.distributed.init_process_group).

This should help reduce time to detect hanging NCCL training jobs from 30 minutes to 10 minutes.

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