new fuse_view_copy pass to merge chains of single-user view_copy nodes (#4019)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4019

There are some chains of view_copy -> view_copy nodes which get introduced when running the AddmmToLinearTransform pass.
We can merge two view_copy nodes if the first has only one user (the second view_copy node).

Reviewed By: jorgep31415

Differential Revision: D58761418

fbshipit-source-id: 6036c66e8cd551124a3a1fb7480283803cb9c82a