Add pass to remove local_scalar_dense (#5886)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5886

## Context

Scalar tensors (i.e. tensors with only 1 element) are often passed in to functions as scalars via

```
scalar_tensor[0].item()
```

This translates to the following chain in the graph

```
index_select = index_select(scalar_tensor, ...)
scalar = local_scalar_dense(index_select)
```

This diff introduces a pass to remove the `local_scalar_dense` "chain" in favor of passing in the input tensor directly.

Note that this replacement only occurs if the original tensor is a scalar tensor.

In the Vulkan backend, these scalar tensors will be represented as symbolic integers instead of actual tensors, which is why this replacement is valid. However, it may not a valid replacement for other backends.
ghstack-source-id: 246752220

Reviewed By: jorgep31415

Differential Revision: D63913432

fbshipit-source-id: 86ab48ef6a171cd631643db521e8dec041fa63e0