allow non-final returns (#15463)

allow non-final returns (#15463)

Summary:
This PR allows a subclass of programs that have return statements that are not final in the graph.

`final_returns.h` contains the a comment describing how this is accomplished.
To minimize complexity in `compiler.cpp`, this pass is done as an AST-to-AST rewrite before the compiler runs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15463

Differential Revision: D13538962

Pulled By: zdevito

fbshipit-source-id: 67105ca873351825b4a364092ab1873779f3e462