[jit] Fix bound method copying (#36546)
Summary:
Previously we were copying the bound method of the original class to the
new script module class, which causes `self` to be wrong. This PR
changes it so we fetch the unbound function, then bind it to the new
script module, then attach it to the module.
Fixes #28280
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36546
Pulled By: driazati
Differential Revision: D21023329
fbshipit-source-id: 6b3f8404700860151792f669a9c02fbd13365272
