[tune] Add Fractional GPU example/docs (#3169)

* Add example for fractional GPU support

* Update tune_mnist_keras.py

* Update doc/source/tune-usage.rst

doc/source/tune-usage.rst

@@ -193,9 +193,12 @@ Using GPUs (Resource Allocation)
 
 Tune will allocate the specified GPU and CPU ``trial_resources`` to each individual trial (defaulting to 1 CPU per trial). Under the hood, Tune runs each trial as a Ray actor, using Ray's resource handling to allocate resources and place actors. A trial will not be scheduled unless at least that amount of resources is available in the cluster, preventing the cluster from being overloaded.
 
+Fractional values are also supported, (i.e., ``"gpu": 0.2``). You can find an example of this in the `Keras MNIST example <https://github.com/ray-project/ray/blob/master/python/ray/tune/examples/tune_mnist_keras.py>`__.
+
 If GPU resources are not requested, the ``CUDA_VISIBLE_DEVICES`` environment variable will be set as empty, disallowing GPU access.
 Otherwise, it will be set to the GPUs in the list (this is managed by Ray).
 
+
 If your trainable function / class creates further Ray actors or tasks that also consume CPU / GPU resources, you will also want to set ``extra_cpu`` or ``extra_gpu`` to reserve extra resource slots for the actors you will create. For example, if a trainable class requires 1 GPU itself, but will launch 4 actors each using another GPU, then it should set ``"gpu": 1, "extra_gpu": 4``.
 
 .. code-block:: python

python/ray/tune/examples/tune_mnist_keras.py

@@ -105,6 +105,8 @@ def create_parser():
     parser = argparse.ArgumentParser(description='Keras MNIST Example')
     parser.add_argument(
         "--smoke-test", action="store_true", help="Finish quickly for testing")
+    parser.add_argument(
+        "--use-gpu", action="store_true", help="Use GPU in training.")
     parser.add_argument(
         '--jobs',
         type=int,
@@ -113,8 +115,8 @@ def create_parser():
     parser.add_argument(
         '--threads',
         type=int,
-        default=None,
-        help='threads used in operations (default: all)')
+        default=2,
+        help='threads used in operations (default: 2)')
     parser.add_argument(
         '--steps',
         type=float,
@@ -185,6 +187,10 @@ def create_parser():
                 },
                 "run": "train_mnist",
                 "num_samples": 1 if args.smoke_test else 10,
+                "trial_resources": {
+                    "cpu": args.threads,
+                    "gpu": 0.5 if args.use_gpu else 0
+                },
                 "config": {
                     "lr": lambda spec: np.random.uniform(0.001, 0.1),
                     "momentum": lambda spec: np.random.uniform(0.1, 0.9),