Constant propagation changes (#16244)

Constant propagation changes (#16244)

Summary:
- remove loop node that is guaranteed not to execute
- remove extra loop outputs that are no longer needed

- if we are inlining an if node, only run constant propagation on the block that will execute

- remove the recurse argument since we only expose the Graph Constant Propagation and it's not used

This also includes  a few extra hooks to python_ir that I think make it a little be easier to test graph conditions from python.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16244

Differential Revision: D13791635

Pulled By: eellison

fbshipit-source-id: d16351fffcfc8013b02015db200f8fde002e0577