Add sample input for a subset of core ATen ops (#262)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/262

Basically move all the sample inputs to functional variants. Then we can generate sample inputs for out variants based on that. This is also aligned with our compiler flow where we lower from core ATen ops to their out variants.

Reviewed By: manuelcandales

Differential Revision: D49088856

fbshipit-source-id: 71105773bdc07e7e0ca850ef7e79b3af7f1d5fa5