[nvFuser] add torch.jit.fuser context manager (#38993)
Summary:
1. `torch.jit.fuser(str)` context manager facilitates switch between backend fusers:
str - 'fuser0' enables only legacy fuser;
str - 'fuser1' enables only NNC;
str - 'fuser2' enables only nvFuser;
2. cleanup updated python tests.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38993
Reviewed By: nairbv, pbelevich
Differential Revision: D21800620
Pulled By: soumith
fbshipit-source-id: 7fe855f5a5b97368e5e84c98c28d04b2e1276c85
