[Dev] remove dead manual_release_grads code path in 1F1B overlap schedule

[Wohox]

What does this PR do ?

Removes dead code in the 1F1B overlap schedule. ScheduleNode.manual_release_grads was hardcoded to False at construction in pipeline_parallel/utils.py and never reassigned anywhere in the codebase, so both if self.manual_release_grads: branches were unreachable. The supporting bookkeeping (delay_grads_release, output_grads) existed only to feed those unreachable branches.

Removed:

• pipeline_parallel/utils.py: self.manual_release_grads = False and self.delay_grads_release = False fields on ScheduleNode; the unreachable g.untyped_storage().resize_(0) branch in _backward.
• models/gpt/fine_grained_callables.py: in backward_impl, the if self.delay_wgrad_compute: self.output_grads = grads; self.delay_grads_release = ... block (only consumed by the dead release path); in backward_dw, the unreachable tensor.untyped_storage().resize_(0) loop, the now-meaningless assert self.delay_grads_release, and the explicit self.output_grads = None clearing.

PyTorch's caching allocator already reclaims the dgrad tensors when the last Python reference drops at the end of _backward / backward_dw, so this whole machinery was a no-op. Same scope as #4122.

Why this is safe

A/B comparison was run with the flag temporarily exposed as a TransformerConfig field, on qwen3_moe_proxy (PP=1 EP=8, overlap_moe_expert_parallel_comm + delay_wgrad_compute enabled, 100 iters on real 8×H100, EOS):

manual_release_grads	Peak max-allocated (MB)
False (status quo)	12479.13
True (release on)	12479.13

Peak allocated memory is identical to two decimals.

⚠️ For major changes (either in lines of code or in its impact), please make sure to first share a design doc with the team. If you're unsure what's the best way to do so, contact the @mcore-oncall.

Companion main PR: #4511

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• I have added relevant unit tests
• I have added relevant functional tests
• I have added proper typing to my code Typing guidelines
• I have added relevant documentation
• I have run the autoformatter.sh on my PR

(Pure deletion of unreachable code; existing 1F1B-overlap functional tests cover the surrounding paths.)

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[Wohox]

/ok to test 43e037e

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