fix(spec_decode): bypass embedding dim check for MTP speculative decoding methods

[ArjunPakhan]

Description

This PR resolves a regression introduced in version 0.25.1 where Gemma4 MTP speculative decoding fails at engine initialization with a RuntimeError: a and b must have same reduction dim, but got [s47, 3840] X [5632, 1024] shape validation exception.

Root Cause

A recent linter safeguard designed to prevent shared text-embedding crashes (introduced during Eagle3/XPU updates) enforces that a draft model must fall back to its own layout if its embedding dimension differs from the target model.

However, Gemma4 MTP architecture intentionally utilizes the target model's embedding size (1,792) and concatenates it with the hidden states (3,840) to form the required input features vector (5,632) entering pre_projection. Because the safeguard inadvertently severed the link to the target's embeddings, the engine only passed the bare 3,840 tensor array down to the linear mapping, causing structural dimension layout failures during compilation and lazy graph execution traces.

Solution

Modified the shared-embedding guard clause in vllm/v1/spec_decode/llm_base_proposer.py to check for and bypass dimension mismatch overrides if the chosen speculative inference framework method is strictly set to "mtp". This restores parity with the original initialization mapping from 0.21.0 while maintaining full stability protection for Eagle3 modules.

Fixes #48848

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring / Documentation / Testing

How Has This Been Tested?

1. Initialized serving node with coder3101/gemma-4-26B-A4B-it-heretic (--quantization fp8) as target and google/gemma-4-26B-A4B-it-assistant as MTP speculator.
2. Verified the engine successfully clears lazy tracing pipelines and triggers runtime deployment parameters under the V1 Model Runner without needing VLLM_USE_V2_MODEL_RUNNER=1 environment flags.

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