Run Muon once per optimizer step under ZeRO-3, not once per micro-batch

[alanhuangyoo]

Fixes #8443.

The problem

ZeRO-3 applies Muon inside the gradient reduce (_apply_distributed_muon_update, called from __avg_scatter_contiguous_grads), and that runs every micro-batch. With gradient_accumulation_steps: n, the momentum advances n times per optimizer step, and Newton-Schulz orthogonalizes each micro-batch's partial gradient instead of the accumulated one. ZeRO-1/2 apply Muon at the accumulation boundary and are correct.

On 2 GPUs, fp32, with the same 8 samples per step either way (one micro-batch of 8 at gas=1, four of 2 at gas=4), three steps, relative difference in the weights:

	master	this PR
ZeRO-2, gas=1 vs gas=4	3.6e-4	3.6e-4
ZeRO-3, gas=1 vs gas=4	1.3e-1	3.6e-4
Newton-Schulz calls, ZeRO-3, 2 matrices, 2 steps, gas=4	16	4

ZeRO-3 now lands on exactly ZeRO-2's figure.

The change

This is option 1 from the discussion in #8443. It is scoped to ZeRO-3 without optimizer offload.

• The reduce path no longer runs Muon when optimizer offload is off. The partitions accumulate the raw averaged gradient, as they do for every other optimizer.
• step() calls _apply_muon_to_accumulated_grads() after the overflow check and before the gradient norm. For each Muon sub-group, it:
  • all-gathers each parameter's accumulated gradient partitions, in chunks bounded by reduce_bucket_size as the reduce buckets were;
  • runs the existing round-robin Muon update once;
  • writes each rank's slice back into its partition.
• The per-sub-group body of _apply_distributed_muon_update is moved into _muon_update_sub_group. It takes the full-shape gradients explicitly, because at step time the parameters are partitioned and param.grad can't hold them. The reduce path calls it with param.grad as before.
• The gradient norm is still taken over the Muon update, as before. Clipping semantics are unchanged (Default gradient_clipping divides every Muon update by its own norm, shrinking the step by a model-sized factor #8439 / Fix Muon optimizer conflict with gradient clipping in ZeRO 1/2 #7776 are separate).
• Because the update now runs after the overflow check, a step the loss scaler discards no longer touches the momentum. That is the ZeRO-3 counterpart of [muon] Keep the momentum out of steps the loss scaler discards #8435.
• Collectives: each Muon parameter's gradient and momentum are gathered once per step instead of once per micro-batch. At gas=n that is n times fewer.

The optimizer-offload path is unchanged; #8464 is working on it. jinyouzhi added a pointer to this shape in #8464, and the overlap is limited to _apply_distributed_muon_update.

Testing

On 2×H20:

• New file tests/unit/v1/ops/muon/test_muon_zero3_grad_accum.py. Both tests fail on master and pass here.
  • test_newton_schulz_runs_once_per_matrix_per_step: Newton-Schulz calls summed over ranks come to 2 × steps, not 2 × steps × gas.
  • test_gradient_accumulation_matches_one_large_micro_batch[2, 3]: gas=1 and gas=4 agree to within half-precision Newton-Schulz noise at both stages.
• tests/unit/v1/ops/muon/ plus tests/unit/runtime/zero/test_per_head_muon.py: 260 passed.