[CI] Fix tests/distributed/test_ca_buffer_sharing.py

[ilmarkov]

Fix #18589
Fix #22840

Changes default parameter of rank argument to None instead of 0 in CustomAllreduce.free_shared_buffer. It has to be None as by default it the method will just get it from dist.get_rank()

$ torchrun --nproc_per_node=2 tests/distributed/test_ca_buffer_sharing.py
Rank 1 has pointers [131710626824192, 131710624727040]
Rank 0 has pointers [125588954152960, 125588956250112]
Rank 0 verified all buffers
Rank 1 verified all buffers

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• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

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[gemini-code-assist]

Code Review

This pull request correctly fixes a bug in CustomAllreduce.free_shared_buffer by changing the default value of the rank parameter from 0 to None. The previous default value caused incorrect behavior in multi-process settings, where ranks other than 0 would attempt to free the buffer for rank 0 if the method was called without an explicit rank. With this change, the method now correctly retrieves the rank of the current process using dist.get_rank() when no rank is provided, which is the intended behavior. The fix is clean, targeted, and resolves the issue effectively.

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[yewentao256]

Thanks for the fix!

[robertgshaw2-redhat]

all failures are known and addressed by other PRs. This is good to merge