Refactor HF _QuantSparseMoe: config-driven token counting, NemotronH detection

[realAsma]

What does this PR do?

Type of change: New feature

Overview: Extend _QuantSparseMoe to support NemotronH-style MoE blocks (which use n_routed_experts instead of num_experts) and refactor the MoE calibration features to be config-driven and lazy-initialized.

Key changes:

• _is_sparse_moe_block in plugins/huggingface.py now accepts n_routed_experts (NemotronH pattern) in addition to num_experts
• _QuantSparseMoe is refactored: token counting and forced expert forwarding are now opt-in via config knobs (moe_calib_experts_ratio, moe_count_expert_calib_tokens). When both are off (default), forward is a zero-overhead pass-through.
• Token counting buffer and gate hook are lazy-initialized on first use instead of eagerly in _setup
• _QuantSparseMoe gets layer_sync_moe_local_experts_amax to sync input quantizer amax across experts (same as Megatron path)
• Extract shared sync_moe_experts_input_amax utility into utils.py, also fixing missing weight amax for experts that received no tokens during calibration. Megatron's _MegatronSequentialMLP now calls this shared utility.
• SequentialQuantizer delegates amax property

Testing

• Updated and added unit tests in test_sparse_moe.py covering default config, lazy init, token counting, top_k restoration, and end-to-end quantize with both features enabled.

Before your PR is "Ready for review"

• Is this change backward compatible?: Yes
• Did you write any new necessary tests?: Yes
• Did you add or update any necessary documentation?: No
• Did you update Changelog?: No

[copy-pr-bot]

Auto-sync is disabled for draft pull requests in this repository. Workflows must be run manually.

Contributors can view more details about this message here.

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

🗂️ Base branches to auto review (3)

• main
• release/.*
• feature/.*

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 95bb3fb9-4aaa-46ed-a202-f7e2caead8ff

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch asma/nemotronh_moe_support

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}