Functions for handling zero dim tensor (#693)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/693

Update `tensor_has_dim` to also account for zero-rank tensors, which allow dim to be 0 or -1.
Also introduced two helper functions to `nonzero_dim` and `nonempty_size` that are used to handle zero-rank tensors
ghstack-source-id: 203341574
exported-using-ghexport

Reviewed By: SS-JIA

Differential Revision: D49735859

fbshipit-source-id: 2ed843a12d922fd78b0f7e67b6ff89cc871298d4