torch/tfjs: Translate select to gather.

[juharris]

Description

Translate PyTorch's gather to Tensorflow.js' gather. I am trying to write a Plan in Python with PyTorch that uses tensor.select and I would like it translated so that I can use syft,js. Did I do this right?

How has this been tested?

Added tests.

Checklist

• [x] I have followed the Contribution Guidelines and Code of Conduct
• [x] I have commented my code following the OpenMined Styleguide
• [x] I have labeled this PR with the relevant Type labels
• [x] My changes are covered by tests

[juharris]

@Nolski I resolved the conflicts.