[Dev][feat] Support overlapping A2A Combine backprop with wgrad GEMM

[Wohox]

What does this PR do ?

PR for main: #3795

Problem

In MoE models, the expert weight gradient (wgrad) computation during backward is serialized on the main CUDA stream. This blocks the data gradient (dgrad) from flowing to earlier layers until the expert wgrad finishes, even though there is no data dependency between them. The result is wasted GPU cycles — earlier layers' backward pass sits idle waiting for expert wgrad to complete.

With FSDP, this is further compounded because the gradient reduce-scatter for expert parameters is also blocked on the same critical path.

Solution

This PR introduces a new flag --delay-wgrad-compute-for-te-grouped-gemm that separates the expert wgrad computation from the main backward stream:

1. Two autograd functions are inserted into the MoE layer's forward graph:

   • _RecordExpertDgradCompletion — placed before the expert computation; during backward, it records a CUDA event once the expert dgrad is done.
   • _RegisterDelayedWgradForExperts — placed at the dispatch boundary; during backward, it waits on the dgrad event, then launches backward_dw() on a dedicated CUDA stream, and synchronizes back to the main stream before proceeding.

2. FSDP integration — When used with MegatronFSDP, expert parameters are marked with _fsdp_delay_grad_reduce = True so the normal post-accumulate-grad hook skips them. A callback is registered via register_process_expert_grads_fn() that triggers the FSDP reduce-scatter for expert parameters only after the delayed wgrad computation completes.

3. TE GroupedLinear is configured with delay_wgrad_compute=True, which tells Transformer Engine to skip wgrad during the normal autograd backward and instead wait for an explicit backward_dw() call.

How to enable

--delay-wgrad-compute-for-te-grouped-gemm

Requirements:

• Transformer Engine >= 2.3.0
• moe_grouped_gemm enabled (not legacy grouped gemm)
• Mutually exclusive with --delay-wgrad-compute (the existing A2A-overlap-based delay)
• Mutually exclusive with --overlap-moe-expert-parallel-comm

Works with both FSDP and 3-D parallelism (TP/EP/PP).

What is achieved

The expert wgrad computation runs on a separate CUDA stream, overlapping with the EP communication within the same transformer layer. This reduces the wall-clock time of the backward pass without changing numerical results — the feature is bit-exact with the non-delayed baseline (verified by unit tests comparing per-step losses and final weights over multiple optimizer steps).

Changes

File	Description
megatron/core/model_parallel_config.py	New config flag delay_wgrad_compute_for_te_grouped_gemm
megatron/core/transformer/transformer_config.py	Validation assertions for the new flag
megatron/core/transformer/moe/moe_layer.py	Autograd functions for delayed wgrad + dedicated CUDA stream/event + register_process_expert_grads_fn callback
megatron/core/extensions/transformer_engine.py	Pass delay_wgrad_compute=True to TE GroupedLinear when the new flag is set
megatron/core/distributed/fsdp/.../megatron_fsdp.py	FSDP hook to defer reduce-scatter for expert params and trigger it after delayed wgrad
tests/unit_tests/a2a_overlap/test_delay_wgrad_compute.py	Unit tests covering basic, shared-expert, multi-layer, and FSDP scenarios

Test plan

• Unit test: test_delay_wgrad_compute_for_te_grouped_gemm — full-model training loop (forward → backward → optimizer) comparing delayed vs. non-delayed across num_layers × shared_experts × dispatcher_type × fp8_flag
• Unit test: test_delay_wgrad_compute_for_te_grouped_gemm_with_fsdp — same comparison with MegatronFSDP wrapping (fully_shard_model + fully_shard_optimizer), verifying the deferred reduce-scatter path

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[Wohox]

/ok to test 48b1c29

[Wohox]

/claude review

[Wohox]

/ok to test adb70af

[Wohox]

/ok to test 6d5d7be

[Wohox]

/ok to test a7d2b2b

[shjwudp]

LGTM, thanks!

[Wohox]

/ok to test 3466356

[Wohox]

/ok to test 9d33e8f

[Wohox]

/ok to test 00bd62e