Set current CUDA device in FlashInfer DeepGEMM path

[fergusfinn]

Base: main (upstream vLLM v0.22.1)

This moves the FlashInfer/DeepGEMM current-device workaround from the launch scripts into the vLLM kernel wrapper. The dynamic FlashInfer DeepGEMM blockscale path now sets the CUDA current device from its input tensor before dispatching either branch.

Validation:

• python3 -m py_compile vllm/model_executor/kernels/linear/scaled_mm/flashinfer.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.