[Model] Complete port of Transformers v5 heterogeneous config fix

[amd-callumm]

Purpose

Port remaining infrastructure changes from upstream PR vllm-project#49797 (commit 70b84f0) to fix Gemma-4 models with Transformers v5. Some changes are adapted/added for the ROCm fork/gfx11 branch.

The partial port (c8e2f26) only covered model files, but the error occurs earlier in model_arch_config_convertor.py during config init.

Changes:

1. Infrastructure changes to ModelArchitectureConfig:

   • Add per_layer_overrides field
   • Add getitem for layer-specific views
   • Add from_layers() classmethod to merge per-layer configs

2. Model arch config convertor updates:

   • Add get_per_layer_hf_configs() to generate layer configs
   • Refactor convert() to handle per-layer configs via from_layers()
   • Override for Gemma4 to handle both Transformers v4 (layer_types)
     and v5 (is_heterogeneous) formats
   • Add get_total_num_kv_heads() override to return max across layers

3. Helper function for layer config access:

   • gemma4_layer_config() handles both v4 and v5 formats
   • Returns per-layer config for given layer_idx

4. Config comparison fix:

   • Use object identity (is not) instead of equality (!=) in
     model.py to avoid triggering eq which accesses all attributes

This enables Gemma-4 to load with both Transformers v4 and v5.

Test Plan

• Wait for the "Build RDNA3/3.5 prototype wheels" job to complete
• Run rocm-scripts nightly regression workflow with all Gemma-4 vLLM tests, using the new vLLM wheel to make sure it passes
• Repeat with ttft-roofline profiling enabled

Test Result

All Gemma-3 nightly regression tests are passing both without profiling and with ttft-roofline profiling using the vLLM wheel built against this PR (previously hit AmbiguousGlobalPerLayerAttributeError on many tests)

[amd-callumm]

The pre-commit lint complaints in .buildkite/ and tests/ appear unrelated to these changes.