[Static Runtime] Fix a bug in aten::index (#66940)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66940
`aten::index`'s schema is as follows:
```
"aten::index.Tensor(Tensor self, Tensor?[] indices) -> Tensor
```
The current implementation assumes `indices`' elements are all tensors by doing `elem.toTensor`, which is incorrectly. This change creates an empty optional value if an element from `indices` is not a tensor.
Test Plan: Fixed `StaticRuntime, IndividualOps_Index` to correctly test `aten::index` with `indices` that contains `None`.
Reviewed By: hlu1
Differential Revision: D31712145
fbshipit-source-id: be1c29674bcd55b67b0dcc2a988bc37fd43745f3
