Add tf.contrib.distribute, which defines classes DistributionStrategy

and MirroredStrategy, and related functionality.

Also add tf.contrib.optimizer_v2, an update to the Optimizer API.

RELNOTES: Can now pass tf.contrib.distribute.MirroredStrategy() to
tf.estimator.RunConfig() to run an Estimator model on multiple GPUs
on one machine.
PiperOrigin-RevId: 190996247