Add is_on_policy property to agents (#480)

rl_coach/agents/ddpg_agent.py

@@ -130,6 +130,10 @@ class DDPGAgent(ActorCriticAgent):
         self.TD_targets_signal = self.register_signal("TD targets")
         self.action_signal = self.register_signal("actions")
 
+    @property
+    def is_on_policy(self) -> bool:
+        return False
+
     def learn_from_batch(self, batch):
         actor = self.networks['actor']
         critic = self.networks['critic']