Cleanup custom op library after each custom_op test (#100980)
This PR tells the custom op tests to destroy all custom ops with
specified namespace after each test.
The general problem is that if a test fails, the custom op isn't cleaned
up. We could fix this via try-finally, but using a tearDown method
seemed like a nice O(1) solution.
Test Plan:
- deleted some foo._destroy, verified that the test suite passes.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100980
Approved by: https://github.com/soulitzer, https://github.com/bdhirsh
