diff --git a/imperative/python/megengine/core/autodiff/grad.py b/imperative/python/megengine/core/autodiff/grad.py
index 4a6114dcf7e6187f241aba12ffd5c4038aab2c6f..805e04370a13b565be7a6aa6ea596f0d85faa73a 100644
--- a/imperative/python/megengine/core/autodiff/grad.py
+++ b/imperative/python/megengine/core/autodiff/grad.py
@@ -123,6 +123,11 @@ class Function(ops.PyOpBase):
 
             This method should return a tuple of Tensor or a single Tensor representing the output
             of the function.
+
+        .. note::
+
+            positional arguments should all be Tensor
+
         """
         raise NotImplementedError
 
diff --git a/imperative/src/impl/op_trait.cpp b/imperative/src/impl/op_trait.cpp
index 640583a671ece68d07e6da9e6d053a1dc8d956c6..59b3241daf597a8967c4caca7e6ec69ff5f70a07 100644
--- a/imperative/src/impl/op_trait.cpp
+++ b/imperative/src/impl/op_trait.cpp
@@ -98,6 +98,12 @@ OpTraitRegistry& OpTraitRegistry::fallback() {
     if (!trait->decide_dispatch_mode) {
         trait->decide_dispatch_mode = fallback_decide_dispatch_mode;
     }
+    if (!trait->make_name) {
+        static auto make_name = [](const OpDef& def) -> std::string {
+            return def.trait()->name;
+        };
+        trait->make_name = make_name;
+    }
     return *this;
 }
 
diff --git a/imperative/src/impl/ops/utility.cpp b/imperative/src/impl/ops/utility.cpp
index 14654134bc96f88dd9be6bc22316f132e9e96c36..f40eef420b3efe2f8c845230292012de39d6322b 100644
--- a/imperative/src/impl/ops/utility.cpp
+++ b/imperative/src/impl/ops/utility.cpp
@@ -18,6 +18,7 @@
 namespace mgb::imperative {
 
 MGB_DYN_TYPE_OBJ_FINAL_IMPL(GenericPyOp);
+OP_TRAIT_REG(GenericPyOp, GenericPyOp).fallback();
 
 namespace { namespace fastpathcopy {
     auto apply_on_var_node(