A round of moving some DistributionStrategy libraries from contrib to

core (just implementations, not public APIs):
  * cross_tower_ops -> cross_device_ops
  * cross_tower_utils -> cross_device_utils
  * input_ops (and test)
  * shared_variable_creator (and test)
  * values

Also:
  * BUILD clean up
  * renaming cross_tower -> cross_device in a number of places.
PiperOrigin-RevId: 221828945