Fix CustomAutogradTest.ReentrantPriority rerun failures (#49581)
Summary:
Clear static variable at the end of the test to ensure test passes after re-runs
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49581
Test Plan:
`./bin/test_api "--gtest_filter=CustomAutogradTest.ReentrantPriority" --gtest_repeat=50`
Before the change all subsequent runs of the test failed with
```
../test/cpp/api/autograd.cpp:681: Failure
Expected equality of these values:
order.size()
Which is: 310
10
```
Reviewed By: mrshenli
Differential Revision: D25632374
Pulled By: malfet
fbshipit-source-id: 4814d22b5dff15e1b38a0187e51070771fd58370
