Makes 'Tensor.gathering' generic over 'Int32' and 'Int64'.

[eaplatanios]

Fixes #259.

[rxwei]

I’m not sure this is the safest solution, because Int8 and Int16 also conform to TensorFlowInteger.

[eaplatanios]

Oh I see. I can make a new protocol then, as you suggested.

[eaplatanios]

It's weird though that the raw ops generation script assigns TensorFlowScalar & BinaryInteger as the signature of the gather ops.

[eaplatanios]

In either case, I created a separate protocol and local tests pass. Currently running CI tests.

[rxwei]

Yeah, I think the wrapper generation script doesn’t pay attention to the specific integer types. We should change the script to use the new protocol for APIs that only take index scalar types.

[eaplatanios]

Sure, I can do that in a separate PR after we're done with this one.

[rxwei]

Thanks!