Add optional per-process DeepGEMM JIT cache

[fergusfinn]

Base: main (upstream vLLM v0.22.1)

This moves the DeepGEMM cache isolation workaround from the launch scripts into vLLM's DeepGEMM wrapper. When DG_JIT_CACHE_PER_PID=1 is set, vLLM writes DeepGEMM JIT artifacts under DG_JIT_CACHE_BASE/pid_, defaulting the base to VLLM_CACHE_ROOT/deep_gemm. Existing behavior is unchanged unless the env var is set.

Validation:

• python3 -m py_compile vllm/utils/deep_gemm.py
• git diff --check

Runtime validation on Isambard is still pending.

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[fergusfinn]

Closing per Fergus: keep vllm-isambard scoped to PR #19 for now and drop these experimental changes from the integration branch.