message: only build decayed lr if lr exists.
diff: @@ -588,7 +588,8 @@ class OptimizerV2(trackable.Trackable):
  return slot_dict[slot_name]
  def _prepare(self, var_list):
  - if var_list:
  + # pre-build the decayed learning rate only if learning rate exists.
  + if var_list and "learning_rate" in self._hyper:
  var_dtypes = set([var.dtype.base_dtype for var in var_list])
  self._decayed_lr_t = {}
  for var_dtype in var_dtypes:
  