Make OL gradient A2A pipeline-triggered (#4498) - #4498
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Summary: ## Context `PECAll2AllSeqWait.backward` eagerly fired the OL gradient AllToAll, so gradient communication was not fully under pipeline control -- which the compute-ahead pipeline needs for overlap and for the deferred-NOL schedule. ## Approach - Remove the eager OL `dist()` from `PECAll2AllSeqWait.backward` so backward only splits the gradient and creates both OL/NOL `PECGradientApply` instances. The pipeline now starts both AllToAlls via `grad_dist`, making gradient communication fully pipeline-controlled (OL before optimizer, NOL deferred to the next batch). - No test change needed: the existing `_pec_backward` already calls `grad_dist(is_overlapped=True)` before waiting OL, so the OL path still triggers `dist()`. ___ Differential Revision: D110148587
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Summary:
Context
PECAll2AllSeqWait.backwardeagerly fired the OL gradient AllToAll, so gradient communication was not fully under pipeline control -- which the compute-ahead pipeline needs for overlap and for the deferred-NOL schedule.Approach
dist()fromPECAll2AllSeqWait.backwardso backward only splits the gradient and creates both OL/NOLPECGradientApplyinstances. The pipeline now starts both AllToAlls viagrad_dist, making gradient communication fully pipeline-controlled (OL before optimizer, NOL deferred to the next batch)._pec_backwardalready callsgrad_dist(is_overlapped=True)before waiting OL, so the OL path still triggersdist().Differential Revision: D110148587