Fix deadlock issues in ThreadPool (#29885)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29885
### Summary
Currently, we have a deadlock issue on iOS when running Resnet50. The problem happens when the task being run in the ThreadPool wants to call `getNumThread()` who will try to acquire the same mutex. And thus cause the deadlock situation. The fix is just remove the guard for `_numThreads`, as it's not likely to change after initialization.
### Test Plan
1. Generate a Resnet50 model using trace_model.py
2. Run `ios/TestApp/bootstrap.sh` to do the benchmark
cc shoumikhin AshkanAliabadi
Test Plan: Imported from OSS
Differential Revision: D18533505
Pulled By: xta0
fbshipit-source-id: 2a069d20b59833ec8b02ff05515c3739a85a15de
