[tune] Fix tutorial training on GPU (#12914)

python/ray/tune/tests/tutorial.py

@@ -93,7 +93,11 @@ def train_mnist(config):
         batch_size=64,
         shuffle=True)
 
+    device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
+
     model = ConvNet()
+    model.to(device)
+
     optimizer = optim.SGD(
         model.parameters(), lr=config["lr"], momentum=config["momentum"])
     for i in range(10):
@@ -161,6 +165,11 @@ space = {
 hyperopt_search = HyperOptSearch(space, metric="mean_accuracy", mode="max")
 
 analysis = tune.run(train_mnist, num_samples=10, search_alg=hyperopt_search)
+
+# To enable GPUs, use this instead:
+# analysis = tune.run(
+#     train_mnist, config=search_space, resources_per_trial={'gpu': 1})
+
 # __run_searchalg_end__
 
 # __run_analysis_begin__