Clean up casting ops (#14947)

Clean up casting ops (#14947)

Summary:
This removes FloatToInt style names replacing it with just the destination
name (e.g. FloatToInt -> Float). This makes it more consistent with the
syntax and makes it easier to add type conversions (just add a new
prim::Int op, for instance).

None of these ops get serialized so this should not effect loading of
old models.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14947

Differential Revision: D13408409

Pulled By: zdevito

fbshipit-source-id: d773fe863f14d9de893f686832769f8cc8903a8e