* explicitly check getitem indices can have at most one ellipsis
previous error with multiple `...`:
```
if index_type not in [None, int, slice, Tensor]: raise IndexError(f"{index_type=} not supported")
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
IndexError: index_type=<class 'ellipsis'> not supported
```
this pr:
```
if len(ellipsis_idx) > 1: raise IndexError("an index can only have a single ellipsis ('...')")
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
IndexError: an index can only have a single ellipsis ('...')
```
* oh we have that already
* test that
* test these
