cleanup static_cast of AutogradMeta (#54103)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54103
The goal is to reduce the spread of static casts in the autograd code as per the comment in https://github.com/pytorch/pytorch/pull/49097#discussion_r543695091
I wasn't sure how to use a virtual method here but a simple method in impl clean it up quite nicely.
Test Plan: Imported from OSS
Reviewed By: agolynski
Differential Revision: D27117840
Pulled By: albanD
fbshipit-source-id: 5f277dde34ccf6bc20f76583b906ff3528cde5aa
