Switch XLA to only override abstract functions (#29438)
Summary:
This is a followup of li-roy 's work https://github.com/pytorch/pytorch/pull/23282. (I messed up the rebase there :(
After https://github.com/pytorch/xla/issues/1225 is done we are good to move the integration to only override abstract functions.
This PR contains a TODO which I'll remove in next 2 followup PRs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29438
Reviewed By: ljk53
Differential Revision: D18399922
Pulled By: ailzhang
fbshipit-source-id: b01761673f519dfb240681180d3f18a4518273ca
