[Python]: Indexing with None and Ellipsis

[kohr-h]

The expectation of Python users familiar with NumPy is that, e.g., array[None, :] and array[..., 0] work, which is not the case currently with NDArray.

Both of these would be quite straightforward to add. I could do that, but I just want to make sure that it's not deliberately left out for some reason.

I'm working with a manual build from a91b364.

[leleamol]

Hi @kohr-h
Thank you for submitting the issue! I'm labeling it so MXNet community members can help address it.
I would highly encourage you to implement the change and create a PR. It will get reviewed by contributors and committers before merging the code.

@mxnet-label-bot [Feature Request, Python, NDArray]