[Bugfix] Resolve B12X modules before Dynamo tracing - #53326

Purpose

Resolve optional B12X modules before TorchDynamo traces model forwards. The B12X accessors previously used functools.cache around importlib.import_module; Dynamo unwraps cached functions and can trace the import during the memory-profile forward. Import attempts now happen when vllm.utils.b12x loads, while runtime accessors perform dictionary lookups only.

This allows B12X linear kernels to run with vLLM model compilation enabled instead of requiring compilation mode NONE.

Duplicate-work checks found no open issue or PR addressing the B12X/Dynamo import path. AI assistance was used to diagnose, implement, and validate this change; the submitter reviewed the resulting diff.

Test Plan

pre-commit run --files \
  vllm/utils/b12x.py \
  tests/model_executor/kernels/test_b12x_linear.py

.venv/bin/python -m pytest -q \
  tests/model_executor/kernels/test_b12x_linear.py::test_b12x_module_lookup_is_dynamo_safe

The regression test calls the B12X accessor from a torch.compile(backend="eager", fullgraph=True) function.

Test Result

• Scoped pre-commit hooks: passed.
• Targeted fullgraph Dynamo regression test: 1 passed.
• Model evaluation was not run because this changes import timing only; kernel selection, tensor operations, and model outputs are unchanged.