[pthreadpool] Set max threadlimit to tsan limit (#89453)
Summary:
This will make sure we don't run into an internal assert for clang tsan which has a cap of 63 on concurrently held lock count.
Seems like it is failing with 64 since the comparison is `<`, so setting it to 63 here.
```
llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_deadlock_detector.h:67 "((n_all_locks_)) < (((sizeof(all_locks_with_contexts_)/sizeof((all_locks_with_contexts_)[0]))))"
```
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Test Plan:
CI
Sandcastle run
Reviewed By: kimishpatel, salilsdesai
Differential Revision: D41444710
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89453
Approved by: https://github.com/mcr229
