[dtensor] enable op db tests by using multithreaded test case (#92198)
Time comparison between using MultithreadedTestCase and MultiProcessTestCase on op db tests is amazing!
using MultiThreadTestCase on a AWS dev node:
```
time pytest test/distributed/_tensor/test_dtensor_ops.py
============= 175 passed, 42 skipped, 397 xfailed in 80.30s (0:01:20) =======
real 1m22.330s
user 1m38.782s
sys 0m18.762s
```
MultiProcessTestCase spends from 40mins to more than 1h, even if using pytest parallel testing tools.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92198
Approved by: https://github.com/XilunWu
