Method returns a single argument (#15289)

Method returns a single argument (#15289)

Summary:
This PR changes Method (just Method not all graphs) to always have a single
return argument.

This is part 1 in a set of changes that will enable us to have better handling if early return statements.
The simplification that this change provides greatly reduces the work for the next step.

This change makes it so that Method and Python handle multiple returns in the same way:
* 0 - None
* 1 - <single value>
* many - Tuple[...]

The result is that a lot of special-case handling in compiler.cpp and its
bindings can be removed. It also fixes several bugs in return handling,
including one where return values were not always checked against their
attributed values.

Notes:
* inferTypeFrom is renamed to be more accurate and discourage use.
* This has uncovered some bugs in other components, which are noted in
  the diff.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15289

Differential Revision: D13481649

Pulled By: zdevito

fbshipit-source-id: 0e2242a40bb28cca2d0e8be48bede96195e4858c