Update `RemoveLocalScalarDenseOpsTransform` to tag scalar tensors as well (#6069)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/6069

## Context

See the new docstrings added to `remove_local_scalar_dense_ops` for more details on what the pass is trying to achieve.

The goal is to mark tensors that are consumed as scalars via `tensor[0].item()` as "scalar tensors" that will be represented as a `SymInt` object in the vulkan delegate instead of a regular `Tensor` object.

This diff also adds an `__init__.py` file to the `_passes` folder to make it easier to include Vulkan passes from one place.
ghstack-source-id: 247163956
exported-using-ghexport

Reviewed By: jorgep31415

Differential Revision: D64139867

fbshipit-source-id: 88ba420e107654d7eadb2cbca78a3750a51f74b0