Allow forward pass without gradient

- previously, a crash would occur if a module with a registered rule
would be passed without requiring gradient
- this commit explicitly allows forward of a module with a registered
rule without requiring gradient computation

zennit/core.py

@@ -143,10 +143,15 @@ def wrapper(grad_input, grad_output):
         if not isinstance(input, tuple):
             input = (input,)
 
-        post_input = Identity.apply(*input)
-        post_input[0].grad_fn.register_hook(wrapper)
-        # work around to support in-place operations
-        post_input = tuple(elem.clone() for elem in post_input)
+        if input[0].grad_fn is not None:
+            # only if gradient required
+            post_input = Identity.apply(*input)
+            post_input[0].grad_fn.register_hook(wrapper)
+            # work around to support in-place operations
+            post_input = tuple(elem.clone() for elem in post_input)
+        else:
+            # no gradient required
+            post_input = input
         return post_input[0] if len(post_input) == 1 else post_input
 
     def post_forward(self, module, input, output):
@@ -160,7 +165,9 @@ def wrapper(grad_input, grad_output):
         if not isinstance(output, tuple):
             output = (output,)
 
-        output[0].grad_fn.register_hook(wrapper)
+        if output[0].grad_fn is not None:
+            # only if gradient required
+            output[0].grad_fn.register_hook(wrapper)
         return output[0] if len(output) == 1 else output
 
     def pre_backward(self, module, grad_input, grad_output):