Improve c2-aten tensor interop and add proper testing (#15860)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15860
Few changes (which are harder to split in separate diffs, so together):
- make conversion explicit (as they can throw to avoid surprises)
- fix tensor legacy dispatch not initialized when tensor is created on C2 side
- add a bunch of invariants to enforce
Reviewed By: ezyang
Differential Revision: D13596031
fbshipit-source-id: d20b601e06ba47aeff2f6e8e15769840e2d46108
