Qualcomm AI Engine Direct - Fix a QNN crash on AMD (pytorch#22543)

### 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](pytorch#22395), 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
pytorch#22542.

Author: qti-horodnic

backends/qualcomm/utils/qnn_sdk_setup.py

@@ -166,7 +166,8 @@ def disable_mkldnn_on_amd() -> None:
 
     import torch
 
-    torch.backends.mkldnn.enabled = False
+    if not torch.backends.flags_frozen():
+        torch.backends.mkldnn.enabled = False
 
 
 def _host_is_amd() -> bool: