message: Change placeholder exception to log instead of throwing an exception.
  It is difficult to remove placeholders from existing meta-graphs without breaking other structures.  As long as the placeholders do not contribute towards the loss/evaluation function they are harmless.
diff: @@ -142,7 +142,8 @@ class TPUReplicateContext(control_flow_ops.XLAControlFlowContext):
  def _AddOpInternal(self, op):
  # pylint: disable=protected-access
  if op.type in _BLACKLISTED_OPS:
  - raise ValueError("Operation of type %s (%s) is not supported on the TPU" %
  + logging.error("Operation of type %s (%s) is not supported on the TPU. "
  + "Execution will fail if this op is used in the graph. " %
  (op.type, op.name))
  if op.type in _NOT_IMPLEMENTED_OPS:
  