[feat] Add Qwen3 MoE true-on-policy parity

[maocheng23]

Summary

Adds the Megatron-LM side of the Qwen3-30B-A3B MoE true-on-policy stack.

This PR is one of three coupled PRs that must land together because they share the qwen3_moe_true_on_policy_v1 contract and were validated as one end-to-end stack.

Companion PRs:

• Miles: [feat] Add Qwen3 MoE true-on-policy parity miles#1059
• SGLang: [feat] Add Qwen3 MoE true-on-policy parity sgl-project/sglang#24408

Target

Bit-identical rollout/train logprob parity for Qwen3-30B-A3B MoE under deterministic decode, with differentiable Megatron backward.

SGLang remains the numerical source of truth. Megatron reproduces the SGLang MoE numerics in training, including deterministic routing/top-k and EP reduction behavior required by the contract.

Main Changes

• Adds the Qwen3-MoE true-on-policy extension path at the MoE layer boundary.
• Uses SGLang fused MoE behavior for the direct parity path and local autograd support for training.
• Updates the SGLang fused MoE imports to the rebased moe_runner.triton_utils layout.
• Keeps the MoE-specific parity path narrow; generic Megatron router/dispatcher fallback edits and debug-only helpers were cleaned out.
• Preserves dense-stack behavior outside the explicit qwen3_moe_true_on_policy_v1 contract.

Validation

Latest rebased EP4 E2E on ion7, Qwen3-30B-A3B, 8x H200, run qwen3-moe-top-ep4-rebased-e2e-260522-ion7-r4:

• Ray job: raysubmit_yc4WK8aCKzP4tx1Q
• Train: TP=1, EP=4, ETP=1, CP=2
• Rollout: rollout_num_gpus=8, rollout_num_gpus_per_engine=4, SGLang TP=4, SGLang EP=4
• Runtime confirmed: recompute_logprobs_via_prefill=False, use_rollout_logprobs=False
• rollout logprob: -2.8848648071289062e-05
• ref logprob: -2.8848648071289062e-05
• train logprob: -2.8848648071289062e-05
• train/train_rollout_logprob_abs_diff = 0.0
• train/train_rollout_kl = 0.0

Local record:
recovery/qwen3_moe_clean/journal/2026-05-22-qwen3-moe-clean-e2e.md

Test Plan

• Rebased onto current radixark/Megatron-LM:miles-main stack.
• Local syntax/diff sanity checks for touched Python paths.
• Remote import/syntax checks in the ion7 container.
• EP4 E2E exact-zero logprob validation on ion7.
• CI replay for the three-PR stack.
• Longer on/off-policy throughput comparison after reviewer signoff on the cleaned deterministic path.