Change basic example to contain backward two losses. (#64)

docs/source/examples/basic_usage.rst

@@ -1,12 +1,14 @@
 Basic Usage
 ===========
 
+This example shows how to use TorchJD to perform an iteration of Jacobian descent on a regression
+model with two objectives. In this example, a batch of inputs is forwarded through the model and two
+corresponding batches of labels are used to compute two losses. These losses are then backwarded
+through the model. The obtained Jacobian matrix, consisting of the gradients of the two losses with
+respect to the parameters, is then aggregated using :doc:`UPGrad <../docs/aggregation/upgrad>`, and
+the parameters are updated using the resulting aggregation.
+
 
-This example shows how to use TorchJD to perform an iteration of Jacobian Descent on a regression
-model. In this example, a batch of inputs is forwarded through the model and the corresponding batch
-of labels is used to compute a batch of losses. These losses are then backwarded through the model.
-The obtained Jacobian matrix, consisting of the gradients of the losses, is then aggregated using
-UPGrad, and the parameters are updated using the resulting aggregation.
 
 Import several classes from ``torch`` and ``torchjd``:
 
@@ -19,7 +21,7 @@ Import several classes from ``torch`` and ``torchjd``:
 
 Define the model and the optimizer, as usual:
 
->>> model = Sequential(Linear(10, 5), ReLU(), Linear(5, 1))
+>>> model = Sequential(Linear(10, 5), ReLU(), Linear(5, 2))
 >>> optimizer = SGD(model.parameters(), lr=0.1)
 
 Define the aggregator that will be used to combine the Jacobian matrix:
@@ -33,28 +35,27 @@ negatively affected by the update.
 Now that everything is defined, we can train the model. Define the input and the associated target:
 
 >>> input = torch.randn(16, 10)  # Batch of 16 input random vectors of length 10
->>> target = input.sum(dim=1, keepdim=True)  # Batch of 16 targets
+>>> target1 = torch.randn(16)  # First batch of 16 targets
+>>> target2 = torch.randn(16)  # Second batch of 16 targets
 
-Here, we generate fake data in which each target is equal to the sum of its corresponding input
-vector, for the sake of the example.
+Here, we generate fake inputs and labels for the sake of the example.
 
 We can now compute the losses associated to each element of the batch.
 
->>> loss_fn = MSELoss(reduction='none')
+>>> loss_fn = MSELoss()
 >>> output = model(input)
->>> losses = loss_fn(output, target)
-
-Note that setting ``reduction='none'`` is necessary to obtain the element-wise loss vector.
+>>> loss1 = loss_fn(output[:, 0], target1)
+>>> loss2 = loss_fn(output[:, 1], target2)
 
-The last steps are similar to gradient descent-based optimization.
+The last steps are similar to gradient descent-based optimization, but using the two losses.
 
 Reset the ``.grad`` field of each model parameter:
 
 >>> optimizer.zero_grad()
 
 Perform the Jacobian descent backward pass:
 
->>> torchjd.backward(losses, model.parameters(), A)
+>>> torchjd.backward([loss1, loss2], model.parameters(), A)
 
 This will populate the ``.grad`` field of each model parameter with the corresponding aggregated
 Jacobian matrix.

tests/doc/test_rst.py

@@ -6,20 +6,21 @@ def test_basic_usage():
     import torchjd
     from torchjd.aggregation import UPGrad
 
-    model = Sequential(Linear(10, 5), ReLU(), Linear(5, 1))
+    model = Sequential(Linear(10, 5), ReLU(), Linear(5, 2))
     optimizer = SGD(model.parameters(), lr=0.1)
 
     A = UPGrad()
-
     input = torch.randn(16, 10)  # Batch of 16 input random vectors of length 10
-    target = input.sum(dim=1, keepdim=True)  # Batch of 16 targets
+    target1 = torch.randn(16)  # First batch of 16 targets
+    target2 = torch.randn(16)  # Second batch of 16 targets
 
-    loss_fn = MSELoss(reduction="none")
+    loss_fn = MSELoss()
     output = model(input)
-    losses = loss_fn(output, target)
+    loss1 = loss_fn(output[:, 0], target1)
+    loss2 = loss_fn(output[:, 1], target2)
 
     optimizer.zero_grad()
-    torchjd.backward(losses, model.parameters(), A)
+    torchjd.backward([loss1, loss2], model.parameters(), A)
     optimizer.step()