Merge with master

python/ray/rllib/agents/impala/vtrace_policy_graph.py

@@ -9,6 +9,7 @@
 import gym
 import ray
 import numpy as np
+import tensorflow as tf
 from ray.rllib.agents.impala import vtrace
 from ray.rllib.evaluation.metrics import LEARNER_STATS_KEY
 from ray.rllib.evaluation.policy_graph import PolicyGraph
@@ -20,9 +21,6 @@
 from ray.rllib.utils.annotations import override
 from ray.rllib.utils.error import UnsupportedSpaceException
 from ray.rllib.utils.explained_variance import explained_variance
-from ray.rllib.utils import try_import_tf
-
-tf = try_import_tf()
 
 # Frozen logits of the policy that computed the action
 BEHAVIOUR_LOGITS = "behaviour_logits"