message: Add EPU device type. Allow passing string to initialize_system_for_tpu_embedding.
diff: @@ -21,7 +21,8 @@ from __future__ import print_function
  from tensorflow.python.util.tf_export import tf_export
  -_VALID_DEVICE_TYPES = frozenset({"CPU", "GPU", "TPU", "CUSTOM"})
  +# EPU represents for TPU embedding for now. Subject to change in future.
  +_VALID_DEVICE_TYPES = frozenset({"CPU", "GPU", "TPU", "CUSTOM", "EPU"})
  # ==============================================================================
  