fix(cp): inherit requires_grad in empty-response redistribute fill#1635
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EazyReal wants to merge 1 commit into
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fix(cp): inherit requires_grad in empty-response redistribute fill#1635EazyReal wants to merge 1 commit into
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When a CP rank owns none of a sample response, allgather_cp_redistribute inserts a zero-fill tensor before its differentiable all-reduce. Hardcoding requires_grad=True gives detached metric and reference-logprob paths different autograd graphs across ranks and can hang the collective. Inherit requires_grad from the input tensor so all ranks agree while preserving gradient flow for loss tensors. Extend the existing CP utility tests with both detached and differentiable cases.
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This pull request modifies allgather_cp_redistribute in cp_utils.py so that the generated fill tensor inherits the requires_grad attribute from the input value tensor instead of defaulting to True. A unit test has also been added to verify this behavior. There are no review comments, so I have no feedback to provide.
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Summary
Make empty-response context-parallel fill tensors inherit requires_grad from the redistributed input.
Why
The detached fill tensor can break backward when a context-parallel shard contains no response tokens. This matches the Slime fix from THUDM/slime#2123.
Testing
pytest -q tests/fast/backends/training_utils/test_ulysses_cp_utils.py