Uses `tune.run` to execute experiments as preferred API.
@noahgolmant
This does not break backwards compat, but will slowly internalize `Experiment`.
In a separate PR, Tune schedulers should only support 1 running experiment at a time.
This PR introduces the following changes:
* Ray Tune -> Tune
* [breaking] Creation of `schedulers/`, moving PBT, HyperBand into a submodule
* [breaking] Search Algorithms now must take in experiment configurations via `add_configurations` rather through initialization
* Support `"run": (function | class | str)` with automatic registering of trainable
* Documentation Changes
