Remove memory-format workaround for Arm backend (#3981)

Summary:
Remove temporary fix for memory format introduced in https://github.com/pytorch/executorch/pull/2371. The dim-order of each node is annotated in a pass. Also some refactoring of arm_backend.py.

Pull Request resolved: https://github.com/pytorch/executorch/pull/3981

Reviewed By: kirklandsign

Differential Revision: D59280091

Pulled By: digantdesai

fbshipit-source-id: f591161830b7c0f836f0be9c33c12c1282f4cc4d