[WIP]: Support mtp fp8

[RunningLeon]

Motivation

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Modification

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BC-breaking (Optional)

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Checklist

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2. The modification is covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
3. If the modification has a dependency on downstream projects of a newer version, this PR should be tested with all supported versions of downstream projects.
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[Copilot]

Pull request overview

This PR updates the PyTorch speculative-decoding (MTP draft) path to better support FP8-weight models by propagating model_format into spec-decode draft model construction and aligning Qwen3.5 MTP module behavior with FP8 checkpoint expectations.

Changes:

• Propagate engine_config.model_format into SpecDecodeConfig.from_config() so draft/MTP models can be built with the same weight-quantization format.
• Adjust Qwen3.5 MTP module prefixing and disable quantization for the MTP fc projection to match FP8 model configs.
• Ensure spec-decode cache config inherits quant_policy from the target cache config.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
lmdeploy/pytorch/models/qwen3_5_mtp.py	Adjust MTP module prefixing and MTP fc quantization behavior for FP8 compatibility.
lmdeploy/pytorch/engine/config_builder.py	Forward model_format into spec-decode draft model config building.
lmdeploy/pytorch/config.py	Thread model_format through SpecDecodeConfig.from_config and carry quant_policy into spec-decode cache config.

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