refactor(mtp): extract BaseMTPModel mixin shared by existing MTP draft models

[sufubao]

What

Extracts a small BaseMTPModel mixin that consolidates the draft-model wiring duplicated across the existing MTP models — deepseek_mtp, mistral_mtp, glm4_moe_lite_mtp, and qwen3_moe_mtp.

Each of those draft models repeated the same setup: reuse the main model's req/mem managers and rope caches, pop the main_model / mtp_previous_draft_models kwargs before the base __init__, and mark themselves as draft models. This moves that shared logic into one place.

Why

A behaviour-preserving cleanup (+8/−99 across the four models) that removes duplication and gives a single home for the is_mtp_draft_model marker. It is a preparatory refactor split out of a larger Qwen3.5 / Qwen3.5-MoE MTP change so the follow-up feature PR stays focused on the new capability.

Changes

• New lightllm/models/base_mtp_model.py — the BaseMTPModel mixin, plus a unit test for its wiring.
• deepseek_mtp / mistral_mtp / glm4_moe_lite_mtp / qwen3_moe_mtp model classes now mix in BaseMTPModel (mixed in before the concrete base model so the shared overrides win via MRO).

Testing

• pre-commit (black 21.12b0 + flake8 6.1.0) clean.
• unit_tests/models/test_base_mtp_model_mixin.py covers the mixin's kwargs-popping, manager/rope sharing, and the is_mtp_draft_model marker.

[gemini-code-assist]

Code Review

This pull request introduces a new BaseMTPModel mixin class to consolidate duplicated initialization logic, manager sharing, and ROPE cache reuse across multiple MTP draft models (DeepSeek, GLM4, Mistral, and Qwen3). The review feedback correctly identifies a critical issue with the cooperative multiple inheritance (MRO) chain in BaseMTPModel._init_custom, which overrides the method without calling super()._init_custom(). Suggestions are provided to call super()._init_custom() and to update the unit tests using a dummy subclass to properly test this cooperative inheritance behavior.

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