Some minor improvements to FakeTensor testing (#106311)
Summary:
- PyTorch testing chokes sometimes when it sees an exception where the first
argument is not a string. fake_tensor.UnsupportedOperatorException's first
arg is an OpOverload. This PR fixes PyTorch testing to not choke. I'm not
really sure how to reproduce this in OSS.
- It turns out that if an operator does not have a meta kernel, the FakeTensor
rule is really slow (30ms in OSS in debug mode, 3s on some internal config).
The thing that is slow (aside from the previous diff) is waiting for the Dispatcher to
report NotImplemented and then attempting to catch that. I'm not really sure
why this is slow but it's easy to workaround so I added a workaround.
Test Plan: - existing tests
Differential Revision: D47917554
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106311
Approved by: https://github.com/eellison
