[rllib] PPO doesn't work with fractional num gpus (#3396)

* frac ppo

* gpu test

python/ray/rllib/optimizers/multi_gpu_optimizer.py

@@ -3,6 +3,7 @@ from __future__ import division
 from __future__ import print_function
 
 import logging
+import math
 import numpy as np
 from collections import defaultdict
 import tensorflow as tf
@@ -44,7 +45,9 @@ class LocalMultiGPUOptimizer(PolicyOptimizer):
         if not num_gpus:
             self.devices = ["/cpu:0"]
         else:
-            self.devices = ["/gpu:{}".format(i) for i in range(num_gpus)]
+            self.devices = [
+                "/gpu:{}".format(i) for i in range(int(math.ceil(num_gpus)))
+            ]
         self.batch_size = int(sgd_batch_size / len(self.devices)) * len(
             self.devices)
         assert self.batch_size % len(self.devices) == 0