Avoid Memory Leak in Rollout worker

ISSUE: When we restore checkpoints, we create new nodes in the
Tensorflow graph. This happens when we assign new value (op node) to
RefVariable in GlobalVariableSaver. With every restore the size of TF
graph increases as new nodes are created and old unused nodes are not
removed from the graph. This causes the memory leak in
restore_checkpoint codepath.

FIX: We reset the Tensorflow graph and recreate the Global, Online and
Target networks on every restore. This ensures that the old unused nodes
in TF graph is dropped.

rl_coach/agents/agent.py

@@ -953,6 +953,12 @@ class Agent(AgentInterface):
         self.input_filter.restore_state_from_checkpoint(checkpoint_dir, checkpoint_prefix)
         self.pre_network_filter.restore_state_from_checkpoint(checkpoint_dir, checkpoint_prefix)
 
+        if self.ap.task_parameters.framework_type == Frameworks.tensorflow:
+            import tensorflow as tf
+            tf.reset_default_graph()
+            # Recreate all the networks of the agent
+            self.networks = self.create_networks()
+
         # no output filters currently have an internal state to restore
         # self.output_filter.restore_state_from_checkpoint(checkpoint_dir)