Added TrainingAlgorithm base class.

Since it seems like the method arguments for training a model are
constant, and because it's convenient for ownership during async
training, this CL adds a base class for training algorithms.

Change-Id: I92059fd49593066e1c68a76eb575b0e07b7fd783
Reviewed-on: https://chromium-review.googlesource.com/c/1423349
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624357}
7 files changed