Add implicit optional unwrapping (#15587)

Add implicit optional unwrapping (#15587)

Summary:
Add support for type inference for optional type refinement.

If a conditional is of the form "x is None" or "x is not None", or is a boolean expression containing multiple none checks, the proper type refinements are inserted in each branch.

For example:
if optional_tensor is not None and len(optional_tensor) < 2:
	# optional_tensor is a Tensor

if optional_tensor1 is not None and optional_tensor2 is not None:
	# both optional_tensor1 and optional_tensor2 are Tensors

TODO:

- not run an op for unchecked unwrap optional in the interpreter

- potentially refine types to prim::None (omitted for now to simply things & because it's not an actual use cause).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15587

Differential Revision: D13733810

Pulled By: eellison

fbshipit-source-id: 57c32be9f5a09ab5542ba0144a6059b96de23d7a