Update CIFAR10 tutorial device selection for CUDA, MPS, and CPU

beginner_source/blitz/cifar10_tutorial.py

@@ -77,13 +77,15 @@
 
 trainset = torchvision.datasets.CIFAR10(root='./data', train=True,
                                         download=True, transform=transform)
+# num_workers=0 avoids multiprocessing spawn issues when running this file as
+# ``python cifar10_tutorial.py`` on macOS/Windows (see note above).
 trainloader = torch.utils.data.DataLoader(trainset, batch_size=batch_size,
-                                          shuffle=True, num_workers=2)
+                                          shuffle=True, num_workers=0)
 
 testset = torchvision.datasets.CIFAR10(root='./data', train=False,
                                        download=True, transform=transform)
 testloader = torch.utils.data.DataLoader(testset, batch_size=batch_size,
-                                         shuffle=False, num_workers=2)
+                                         shuffle=False, num_workers=0)
 
 classes = ('plane', 'car', 'bird', 'cat',
            'deer', 'dog', 'frog', 'horse', 'ship', 'truck')
@@ -292,35 +294,38 @@ def forward(self, x):
 ########################################################################
 # Okay, so what next?
 #
-# How do we run these neural networks on the GPU?
+# How do we run these neural networks on a GPU or other accelerator?
 #
 # Training on GPU
 # ----------------
-# Just like how you transfer a Tensor onto the GPU, you transfer the neural
-# net onto the GPU.
+# Just like how you transfer a Tensor onto a device, you transfer the neural
+# net onto that device.
 #
-# Let's first define our device as the first visible cuda device if we have
-# CUDA available:
-
-device = torch.device('cuda:0' if torch.cuda.is_available() else 'cpu')
-
-# Assuming that we are on a CUDA machine, this should print a CUDA device:
-
+# Let's first select a device. This picks the fastest available accelerator,
+# or falls back to CPU (same pattern as ``quickstart_tutorial.py``).
+device = (
+    torch.accelerator.current_accelerator().type
+    if torch.accelerator.is_available()
+    else "cpu"
+)
+
+# ``device`` is a string such as ``"cuda"``, ``"mps"``, or ``"cpu"``, which you
+# can pass to ``Tensor.to(...)`` and ``nn.Module.to(...)``.
 print(device)
 
 ########################################################################
-# The rest of this section assumes that ``device`` is a CUDA device.
+# The snippets below show how to move the model and batch data to ``device``.
 #
 # Then these methods will recursively go over all modules and convert their
-# parameters and buffers to CUDA tensors:
+# parameters and buffers to tensors on ``device``:
 #
 # .. code:: python
 #
 #     net.to(device)
 #
 #
 # Remember that you will have to send the inputs and targets at every step
-# to the GPU too:
+# to ``device`` too:
 #
 # .. code:: python
 #