code: def apply_step(self, variables, deltas):
          
          if len(variables) != len(deltas):
              raise TensorForceError("Invalid variables and deltas lists.")
          return tf.group(
              *(tf.assign_add(ref=variable, value=delta) for variable, delta in zip(variables, deltas))
          )
docs: Applies the given (and already calculated) step deltas to the variable values.
  
          Args:
              variables: List of variables.
              deltas: List of deltas of same length.
  
          Returns:
              The step-applied operation. A tf.group of tf.assign_add ops.