Assert that there are no None arguments to backwards (#96300)

This assert would have caught #96219

Signed-off-by: Edward Z. Yang <ezyang@meta.com>
Pull Request resolved: #96300
Approved by: https://github.com/bdhirsh

torch/_functorch/aot_autograd.py

@@ -2356,6 +2356,7 @@ def backward(ctx, *flat_args):
 
             def call_compiled_backward():
                 if CompiledFunction.compiled_bw is None:
+                    assert all(a is not None for a in all_args)
                     if aot_config.dynamic_shapes:
                         all_args_list = list(all_args)
                         CompiledFunction.compiled_bw = create_aot_dispatcher_function(