[hw-agnostic] Adding embedding layer and lm_head

[bohnstingl]

Purpose

Extends the hardware-agnostic layer path in the Transformers modeling backend to
cover the token-embedding boundary: input embedding (VocabParallelEmbedding),
LM head (ParallelLMHead), and the logits projection (LogitsProcessor).

When VLLM_USE_HW_AGNOSTIC is unset (the default), behavior is unchanged: every
site resolves to the same vLLM class it used before.

Test Plan

I ran an lm_eval with the Granite3.3-8B model, to demonstrate that the new additional layers do not introduce numerical issues.

export VLLM_USE_HW_AGNOSTIC=1
lm_eval --model vllm \
  --model_args "pretrained=ibm-granite/granite-3.3-8b-instruct,model_impl=transformers,tensor_parallel_size=1,gpu_memory_utilization=0.85,max_model_len=4096,enforce_eager=True" \
  --tasks gsm8k \
  --num_fewshot 5 \
  --batch_size auto \
  --output_path /block/boh/vllm_dev/claude/granite_eval_logs/flag_on

In addition, I also added some new, specific tests for the layers.

Test Result

lm_eval results:

Metric	OFF	ON	Δ
flexible-extract	0.7240 ± 0.0123	0.7225 ± 0.0123	−0.0015
strict-match	0.6710 ± 0.0129	0.6664 ± 0.0130	−0.0046

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

cc @tdoublep @hmellor @zou3519

[claude]

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