What-If Tool code moved to its own repo (out of TensorBoard)

[James Wexler]

TLDR: Find the What-If Tool code at https://github.com/pair-code/what-if-tool

The What-If Tool codebase has been moved from its old home inside of the TensorBoard repo's plugins directory to its own repo. This has a number of benefits, including:

- Easier for people to find, and contribute to, the codebase as it doesn't involve wading around in a much bigger repository.

- The What-If Tool release cycle can now be completely managed by the team, as opposed to the old situation where the witwidget pip package and the version in TensorBoard could be out of sync.

This repo previously only hosted the webpage content for https://pair-code.github.io/what-if-tool/ (on the gh-pages branch), but now also hosts the actual source code on the master branch.

We also updated the directory and file names to no longer reference "interactive inference", which was the name for the What-If Tool before we decided on its great name.

For now, the old What-If Tool code hasn't been deleted from the TensorBoard repository, while we switch TensorBoard's use of the What-If Tool over to their "dyanmic plugin" system which will instead use the code from the what-if-tool repository. All future updates to the code and corresponding pip package releases will come from the code in the new repo, and the old code will be deleted from the TensorBoard repository as soon as we can switch over to enabling the What-If Tool through TensorBoard's dynamic plugin system.

Check out the development guide at https://github.com/PAIR-code/what-if-tool/blob/master/DEVELOPMENT.md with instructions on how to build and contribute to the What-If Tool in this new repository. There will be no noticeable changes to the What-If Tool to users as a result of this move (besides what webpage opens when a user clicks on the documentation link in the top-right of the tool, as of the next release).

Reach out with any questions or issues with the new repo. Thanks!