Tests run on Docker
This PR updates the GPU-based tests to run on docker images. This:
- Simplifies maintenance
- Horizontal scaling is made much simpler
- Environment can be managed with a single line change (the docker image) hello pytorch 1.3-1.7 tests!
- Setups a notification service to get alerted when scheduled tests fail.


Co-authored-by: Morgan <funtowiczmo@gmail.com>
@stas00 I hear you regarding the installation of dependencies. We are installing every dependency except the main one (PT/TF) on the GPU machine, so I would say most errors are caught.
Furthermore, we are installing these exact dependencies on the CircleCI runs, so I believe we are testing that already on every commit.

If there is a scenario I am missing, please let me know and I will do my best to adjust.

All other comments have been adressed in the last three commits.<|||||>Ah, ok, for some reason I thought that if we are using a docker image then we can skip wasting time and resources on installing the same things million times a day and just run the tests right away, and in that case only an occasional test that installs from scratch will be needed. But perhaps this further speed up can be done in some future iteration.

Nothing more from my side.

Thank you, @LysandreJik 