add typing to pybindings (#120)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/120

.pyi file is needed for pyre to recognize the pybindings. Additionally you have to name the .pyi exactly the name of the pybinding lib which is problematic for us because we have 4 libs and 2 arent public. To make matters worse the types arg in the buck rule doesnt handle targets in the same manner as srcs. Put all this together and you get the weird genrule logic in the build files.

A couple of pyre-ignores remain in the test due to the program/operator meta apis that arent in the .pyi because Im going to move them out of pybindings extension

Reviewed By: cccclai

Differential Revision: D48634638

fbshipit-source-id: f7dfb2c63a5231d768d2229ac79c897c64cfc688