Adding a minimal test suite to catch any regressions from accidentally adding backend imports (e.g. `torch`, `tensorflow`, `horovod`) to the main import path.
**Example:** If I'm running Ray Train with `tensorflow`, I should not be required to have `torch` installed.
Currently we install OpenSSH on the fly in fake multinode docker testing. Instead we can speed testing up a fair bit by building a Docker image which includes OpenSSH first and then run tests with this image.
Following #18987 this PR adds a docker-compose based local multi node cluster.
The fake multinode docker comprises two parts. The docker_monitor.py script is a watch script calling docker compose up whenever the docker-compose.yaml changes. The node provider creates and updates the docker compose according to the autoscaling requirements.
This mode fully supports autoscaling and comes with test utilities to start and connect to docker-compose autoscaling environments. There's also a sample test case showing how this can be used.
