Handle Advanced Indexing in ATen

[zdevito]

@killeent @colesbury @gchanan This issue is just to track the progress on advanced indexing plans in ATen. To summarize our discussion, the initial idea is to add 'low-level' support for advanced indexing in the form of an operator like:

Tensor index(const char * format, TensorList inputs);

format is a string that describes indexing, ellispes, and colons: ".tt:", describes a indexing like [...,a_broadcastable_tensor, another_tensor,:] in python. Use of a 't' consumes one entry in inputs.

We will probably need to eventually support:

• t a tensor, in TensorList
• . ellispsis in python ...
• : colon in python :
• r a range specifier a:b, encoded as a tensor with 2 (3?) elements.

This will be the version of indexing wrapped by other interfaces, like PyTorch. We may eventually create a wrapper on this that uses objects more idomatic to C++.