Merge pull request #799 from wanghaoshuang/fix_policy_gradient

Adapt usage of reduce_mean to the latest fluid API.

fluid/policy_gradient/brain.py

@@ -45,7 +45,7 @@ class PolicyGradient:
             label=acts)  # this is negative log of chosen action
         neg_log_prob_weight = fluid.layers.elementwise_mul(x=neg_log_prob, y=vt)
         loss = fluid.layers.reduce_mean(
-            x=neg_log_prob_weight)  # reward guided loss
+            neg_log_prob_weight)  # reward guided loss
 
         sgd_optimizer = fluid.optimizer.SGD(self.lr)
         sgd_optimizer.minimize(loss)