Add int16 support to aten_bridge (#3069)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3069

Running an executorch program via pybindings requires the aten_bridge. This currently fails if the model uses the `int16` dtype. This diff adds support for the type by adding it to the conversion switch statements.

Reviewed By: tarun292

Differential Revision: D56199304

fbshipit-source-id: 19a6815cf2885dda72febf247c3ca3bde91193a8