Qualcomm AI Engine Direct - Fix a QNN crash on AMD

[qti-horodnic]

Summary

Fixed a crash on AMD hosts when PyTorch's global backend flags are frozen.

QnnQuantizer.__init__ calls disable_mkldnn_on_amd(), which assigns torch.backends.mkldnn.enabled = False. PyTorch forbids that assignment once disable_global_flags() has been called, which happens whenever torch.testing._internal.common_utils is imported — as the Qualcomm test suite does. The result is that constructing a quantizer on an AMD host fails with:

RuntimeError: not allowed to set torch.backends.mkldnn flags after disable_global_flags;
please use flags() context manager instead

The assignment is now guarded, so a frozen-flags environment leaves the existing MKLDNN setting in place instead of raising. Disabling MKLDNN is a workaround for AMD-host crashes, not a correctness requirement, so skipping it when the flag is locked is safe.

Note that this is a temporary workaround for an issue introduced in a previous pr, not a root-cause fix, I haven't validated that this change has not broken anything else.

Test plan

Ran the tests mentioned in the test plan section of #22542.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/22543

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❌ 3 New Failures, 1 Unclassified Failure

As of commit 2053ea8 with merge base 843f77e ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / linux / linux-job (gh)
  pytorch_tokenizers
• pull / unittest-buck / linux / linux-job (gh)
  pytorch_tokenizers
• pull / unittest-editable / linux / linux-job (gh)
  pytorch_tokenizers

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• periodic (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)

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[qti-horodnic]

@pytorchbot label "release notes: qualcomm"