Update on "[Executorch] parallelize op_choose_qparams"

When doing prefill for quantized kv cache, with large prefill length, parallelizing this op helps.

Differential Revision: [D84962234](https://our.internmc.facebook.com/intern/diff/D84962234/)

**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D84962234/)!

[ghstack-poisoned]

Author: kimishpatel

extension/llm/custom_ops/TARGETS

@@ -60,5 +60,6 @@ runtime.python_test(
     ],
     deps = [
         "//caffe2:torch",
+        "//executorch/extension/pybindings:portable_lib",
     ],
 )

extension/llm/custom_ops/test_quantized_sdpa.py

@@ -12,6 +12,7 @@
 import torch.nn.functional as F
 
 from executorch.extension.llm.custom_ops import custom_ops  # noqa
+from executorch.extension.pybindings.portable_lib import _unsafe_reset_threadpool
 
 
 def is_fbcode():
@@ -40,6 +41,11 @@ def setUp(self):
         self.q_shape = None
         self.kv_shape = None
         self.is_seq_at_dim_2 = True
+        # For some reason 4 threads doesnt work
+        # This setting is needed to make this test not flaky due to OMP
+        # error of "OMP: Error #131: Thread identifier invalid"
+        # Not clear why that happens but having smaller threadpool resolves it
+        _unsafe_reset_threadpool(3)
 
     def _scale_tensor(self, tensor, min_value, max_value, scale=True):
         normalized_tensor = (tensor - tensor.min()) / (tensor.max() - tensor.min())