Split off fuser tests in test_jit.py to their own test case (#15072)
Summary:
This PR creates TestFuser inside test_jit.py to be a home for graph fuser
specific tests.
This was a useful exercise because now that all the fuser tests are in
one place, I can spot redundant and bitrotting tests for cleanup in a
future PR.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15072
Differential Revision: D13421458
Pulled By: zou3519
fbshipit-source-id: 80b1a7712feff75a0c186d1664601c4edbbca694
