Add MTP speculative-decoding support to NeMo SpeechLM vLLM plugin

[Slyne]

• register the NeMo SpeechLM MTP draft model and route compatible repeated-layer checkpoints through vLLM speculative decoding
• fuse audio embeddings at placeholder positions for target and draft models
• load MTP weights while handling SpeechLM checkpoint prefixes and vocabulary padding
• make plugin registration idempotent and validate repeated-layer multi-head configurations
• add focused plugin tests for registration, config routing, embeddings, and MTP properties

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What does this PR do ?

Add MTP speculative decoding support for NeMo SpeechLM models served through vLLM.

Collection: SpeechLM (nemo.collections.speechlm2)

Changelog

• Register a NeMo SpeechLM MTP draft-model architecture with vLLM.
• Route SpeechLM checkpoints with MTP configuration through vLLM speculative decoding.
• Support repeated-layer, multi-head MTP checkpoints.
• Fuse audio embeddings into placeholder positions for both target and draft models.
• Load MTP checkpoint weights while handling SpeechLM prefixes and vocabulary padding.
• Reject incompatible multi-head checkpoints that were not trained with a repeated MTP layer.
• Make vLLM plugin registration idempotent.
• Add unit tests for registration, configuration routing, weight handling, and multimodal MTP embeddings.

Usage

• You can potentially add a usage example below

from nemo.collections.speechlm2.vllm.salm import register

register()

And then start vllm server:

vllm serve /path/to/speechlm-checkpoint \
  --trust-remote-code \
  --speculative-config '{"method":"mtp","num_speculative_tokens":4}'

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• New Feature
• Bugfix
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Additional Information

• 55 focused SpeechLM vLLM plugin tests.
• vLLM 0.23.0.
• A four-head repeated-layer MTP checkpoint.
• The complete 83,212-sample Open-ASR leaderboard.
• MTP and no-MTP WER remained equivalent within normal run-to-run variation.

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