[dy2static] fix the speed problem introduced by #50883 (#51606)

python/paddle/jit/dy2static/function_spec.py

@@ -373,7 +373,14 @@ def convert_to_input_spec(inputs, input_spec):
             )
         real_spec.name = input_spec.name
         if spec_greater(input_spec, real_spec):
-            return input_spec
+            # change shape but keep the others (stop_gradient / dtype) .
+            real_spec.shape = input_spec.shape
+        else:
+            logging_utils.warn(
+                "input spec is not compatitable with real inputs. input_spec: {input_spec} , real_spec: {real_spec} ".format(
+                    input_spec=input_spec, real_spec=real_spec
+                )
+            )
         return real_spec
     else:
         # NOTE(Aurelius84): Support non-Tensor type as input spec info
@@ -480,8 +487,4 @@ def spec_greater(first, other):
                 return False
         return True
 
-    return (
-        other.stop_gradient == first.stop_gradient
-        and other.dtype == first.dtype
-        and _shape_greater(first.shape, other.shape)
-    )
+    return _shape_greater(first.shape, other.shape)