make magic methods work with casts too (#20654)
Summary:
Previous implementation of magic methods extended from BuiltinOperators, but it should be able to work with other sugared values, such as casts.
I was also considering making CastValue's and BuiltinOperators's extend from a MagicMethod super class, and having them try to call into the super's before their own call. However, not all Builtin Operators have corresponding magic methods so i did it this way instead (although there are workarounds for that).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20654
Differential Revision: D15434469
Pulled By: eellison
fbshipit-source-id: 813fa00bf8b5b9ada46505075ebf984d8eee6aef
