Prevent module full_backward_hook from erroring in double backward (#…

…88357)

Also clarifies documentation to say "execute if and only if gradients wrt outputs are computed" (previously, "execute every time gradients wrt inputs are computed")

See https://docs.google.com/document/d/1tFZKYdsSzRBJ7Di7SWt8X8fSg-E3eiUPwomMF10UyhM/edit for more details regarding the question: 'should module full_backward_hooks be called every time the gradients wrt module inputs are called, or should module full_backward_hooks only be called when the "backward for the module" have been computed?'

Fixes #88312

Pull Request resolved: #88357
Approved by: https://github.com/albanD

test/test_autograd.py

@@ -6638,6 +6638,25 @@ def forward(self, x):
                 gc.collect()
                 self.assertIsNone(ref_())
 
+    def test_full_backward_hook_double_backward(self):
+        x = torch.rand(1, requires_grad=True)
+        y = torch.rand_like(x)
+
+        func = torch.nn.MSELoss()
+        counter = [0]
+
+        def hook(module, grad_input, grad_output):
+            counter[0] += 1
+
+        func.register_full_backward_hook(hook)
+
+        f = func(x, y)
+
+        (gradx_f,) = torch.autograd.grad(f, x, create_graph=True)
+        self.assertEqual(counter[0], 1)
+        _ = torch.autograd.grad(gradx_f, x)
+        # We should not error, and counter should not be incremented
+        self.assertEqual(counter[0], 1)
 
     def test_input_buffer_accum(self):
         leaf = torch.rand(2, 2, requires_grad=True)

torch/nn/modules/module.py

@@ -307,8 +307,10 @@ def register_module_full_backward_hook(
         This adds global state to the `nn.module` module
         and it is only intended for debugging/profiling purposes.
 
-    The hook will be called every time the gradients with respect to module
-    inputs are computed. The hook should have the following signature::
+    The hook will be called every time the gradients with respect to a module
+    are computed, i.e. the hook will execute if and only if the gradients with
+    respect to module outputs are computed. The hook should have the following
+    signature::
 
         hook(module, grad_input, grad_output) -> Tensor or None
 
@@ -1197,8 +1199,10 @@ def register_full_backward_hook(
     ) -> RemovableHandle:
         r"""Registers a backward hook on the module.
 
-        The hook will be called every time the gradients with respect to module
-        inputs are computed. The hook should have the following signature::
+        The hook will be called every time the gradients with respect to a module
+        are computed, i.e. the hook will execute if and only if the gradients with
+        respect to module outputs are computed. The hook should have the following
+        signature::
 
             hook(module, grad_input, grad_output) -> tuple(Tensor) or None
 

torch/utils/hooks.py

@@ -99,13 +99,10 @@ def _unpack_none(self, indices, values):
     def _set_user_hook(self, grad_fn):
         def hook(grad_input, _):
             if self.grad_outputs is None:
-                raise RuntimeError("Module backward hook for grad_input is called before "
-                                   "the grad_output one. This happens because the gradient "
-                                   "in your nn.Module flows to the Module's input without "
-                                   "passing through the Module's output. Make sure that the "
-                                   "output depends on the input and that the loss is computed "
-                                   "based on the output.")
-
+                # This happens because the gradient in your nn.Module flows to
+                # the Module's input without " passing through the Module's
+                # output, e.g. when you're doing double backward.
+                return
             res = self._pack_with_none(self.input_tensors_index, grad_input, self.n_inputs)
 
             for hook in self.user_hooks: