fix(imperative/amp): fix distributed backward callback for nhwc amp

GitOrigin-RevId: 4d725b0ea438d078f1a57c1f58dc707e96f314c5

imperative/src/impl/transformations/format.cpp

@@ -379,8 +379,11 @@ ValueRefList concat_rule(
 ValueRefList identity_rule_helper(
         const OpDef& op, const Span<ValueRef>& inputs, const FormatTransformation& t) {
     // mgb_assert(inputs.size() == 1);
-    auto& src = inputs[0].cast(t.value_type());
-    return t.wrap_outputs(imperative::apply(op, t.unwrap_inputs(inputs)), src.format());
+    if (auto& src = inputs[0].as_ref(t.value_type())) {
+        return t.wrap_outputs(imperative::apply(op, t.unwrap_inputs(inputs)), src->format());
+    } else {
+        return t.wrap_outputs(imperative::apply(op, t.unwrap_inputs(inputs)));
+    }
 }
 
 ValueRefList batchnorm_rule(