don't error out in empty_cache under mempool context

[ngimel]

Now instead of erroring out on empty_cache call during graph capture or under mempool context, we will just silently do nothing. This used to be the behavior for mempools, cudagraphs used to error out, but it's fine to just ignore the call.

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 0be0650 with merge base 1f1f229 ():

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / cuda12.8-py3.10-gcc9-sm75 / test (pr_time_benchmarks, 1, 1, linux.g4dn.metal.nvidia.gpu, unstable) (gh) (#153987)
  MISSING REGRESSION TEST

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[ngimel]

@pytorchbot merge -i

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[huydhn]

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[huydhn]

@pytorchbot cherry-pick --onto release/2.8 --fixes vllm-project/vllm#20358 -c regression

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Cherry picking #158152

The cherry pick PR is at #158180 and it is linked with issue vllm-project/vllm#20358. The following tracker issues are updated:

• [v.2.8.0] Release Tracker #156745 (comment)

Details for Dev Infra team

Raised by workflow job