move modules to ops (#257)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/257

Decrease the supported module surface. For ops that are canonical (not decomposed) and single op partitions, we can always partition just using the operator rather than the module (It is difficult to maintain every variant of the op's module, and if the op is used in a larger module like multihead attention then it is impossible to partition using source_fn)

In the future, we will want to only support module which require recomposition or some module level recomposition

Reviewed By: manuelcandales

Differential Revision: D49111303

fbshipit-source-id: eb46aa51cd129200fa3ecd45be34675ab0fa0a0e