[JIT] Fix archive file extension in examples and docs (#50649)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50649
**Summary**
Tutorials, documentation and comments are not consistent with the file
extension they use for JIT archives. This commit modifies certain
instances of `*.pth` in `torch.jit.save` calls with `*.pt`.
**Test Plan**
Continuous integration.
**Fixes**
This commit fixes #49660.
Test Plan: Imported from OSS
Reviewed By: ZolotukhinM
Differential Revision: D25961628
Pulled By: SplitInfinity
fbshipit-source-id: a40c97954adc7c255569fcec1f389aa78f026d47
