Fix tensor names

ml-agents/mlagents/trainers/common/nn_policy.py

                     hidden_policy,
                     self.act_size[0],
                     activation=None,
-                    name="log_std",
+                    name="log_sigma",
-                    "log_sigma_squared",
+                    "log_sigma",
                     [self.act_size[0]],
                     dtype=tf.float32,
                     initializer=tf.zeros_initializer(),