Backward hooks are called during the backward pass. They can be installed withregister_full_backward_pre_hook() and register_full_backward_hook().These hooks will be called when the backward for this Module has been computed.register_full_backward_pre_hook() will allow the user to access the gradients for outputswhile register_full_backward_hook() will allow the user to access the gradientsboth the inputs and outputs. Alternatively, they can be installed globally for all modules withregister_module_full_backward_hook() andregister_module_full_backward_pre_hook().