clean up imports for tensor.py (#48964)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48964
Stop importing overrides within methods now that the circular dependency is gone, and also organize the imports while I'm at it because they're a jumbled mess.
Test Plan: Existing unit tests. Benchmarks are in #48966
Reviewed By: ngimel
Differential Revision: D25590730
Pulled By: robieta
fbshipit-source-id: 4fa929ce8ff548500f3e55d0475f3f22c1fccc04
