Update on "Prevent module full_backward_hook from erroring in double …

…backward"


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


[ghstack-poisoned]

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. Note that the hook will not execute if the gradients with
+    respect to module outputs are not 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. Note that the hook will not execute if the gradients with
+        respect to module outputs are not computed. The hook should have the following
+        signature::
 
             hook(module, grad_input, grad_output) -> tuple(Tensor) or None