[PyTorch Mobile]Move train related files to their own folder (#58205)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58205
It's worthing moving train related files into their own folder since we are adding more code under the mobile directory.
This diff does that.
Test Plan: run unit tests and ci
Reviewed By: iseeyuan
Differential Revision: D28402432
fbshipit-source-id: cd76a1c4f8ff06508cdc3aad8a169fbf34bb4995
