Add preliminary Muon+M-FSDP support#4486
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Route the emerging-optimizer factory through a Megatron-FSDP-specific
path when `ddp_config.use_megatron_fsdp` is set. Megatron-FSDP attaches
grads via `finish_grad_sync()` on DTensor params instead of via DDP's
main_grad buffers, so the standard `Float16OptimizerWithFloat16Params`
wrapper does not apply; we always wrap with `FP32Optimizer` instead and
drive the FSDP step contract from a thin `FSDPMuonChainedOptimizer`
adapter that calls `finish_grad_sync()` and
`install_optimized_model_weights()` around the inner step.
For now this supports ZeRO-0 ("no_shard") only; ZeRO-1/2/3 will work
without errors on the wiring but require a sharding-aware Muon variant
for numerical correctness, added in a follow-up.
Also patch `LayerWiseDistributedOptimizer._allgather_helper` to read
DTensor-backed params via `_local_tensor`, so the layer-wise + FSDP
combination can flatten the local shard rather than the global DTensor.
Add `FSDPZeROTensorParallelMuon`, a TensorParallelMuon subclass that: 1. Extracts the `Shard(0)` local tensor from each gradient DTensor: (`finish_grad_sync` produces a row-shard per DP rank for `optim`, `optim_grads` and `optim_grads_params`). 2. Allgathers the shards across the DP group to reconstruct the TP-local, DP-full gradient matrix. 3. Trims FSDP bucket-padding rows using the DTensor's declared global shape. 4. Delegates Newton-Schulz to the parent class (which handles the TP dimension via `newton_schulz_tp`). 5. Re-shards the orthogonalized result back to a `Shard(0)` DTensor with matching placements so the in-place update in `OrthogonalizedOptimizer.step` does not promote to `Replicate` and trip the global-shape check. The FSDP factory in `_build_megatron_fsdp_emerging_optimizer` now picks `FSDPZeROTensorParallelMuon` for any sharded inner-DP strategy and passes `pg_collection.dp_cp` for dense params and `pg_collection.expt_dp` for expert params (since expert grads reduce-scatter over a different group). "no_shard" continues to use plain `TensorParallelMuon`. DTensor is imported at module scope with a `_HAVE_DTENSOR` guard so the isinstance checks stay cheap and the module still imports on stacks without `torch.distributed.tensor`.
Three phases of tests for the Muon + Megatron-FSDP integration: - Phase 1: `FSDPMuonChainedOptimizer` adapter (single-rank, mock-based). Verifies the step contract – finish_grad_sync -> inner step -> install_optimized_model_weights – and attribute delegation. - Phase 2: `FSDPZeROTensorParallelMuon.orthogonalize` (multi-rank). Asserts the allgather -> Newton-Schulz -> reshard cycle is numerically equivalent to running NS on the full gradient and extracting the local row-shard, including FSDP padding edge cases. Includes a DTensor round-trip test that catches the `p.add_(orthogonalized_dtensor)` placement-promotion bug. - Phase 3: `_build_megatron_fsdp_emerging_optimizer` factory. Confirms the factory dispatches plain `TensorParallelMuon` for `no_shard` and `FSDPZeROTensorParallelMuon` for sharded strategies, and that expert vs. non-expert Muon instances receive `expt_dp` vs. `dp_cp` as their allgather group.
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Introduce Muon support to M-FSDP. Currently 1.5×–2.7× as slow compared to an Adam baseline with a 1B–8B DeepSeek-V3 proxy model. Peak memory slightly lower than with Adam (4–7 % less).