[AMP OP&Test] Fix fp16 check_grad when user_defined_grads is not None (#51959)

* [AMP OP&Test] Fix fp16 check_grad when user_defined_grads are not None

* fix cond

python/paddle/fluid/tests/unittests/op_test.py

@@ -2407,6 +2407,17 @@ class OpTest(unittest.TestCase):
         if numeric_place is None:
             numeric_place = place
 
+        if user_defined_grads is None and self.is_fp16_compared_with_fp32():
+            self.enable_cal_ref_output()
+            numeric_grads = self._get_gradient(
+                inputs_to_check,
+                place,
+                output_names,
+                no_grad_set,
+                user_defined_grad_outputs,
+            )
+            self.disable_cal_ref_output()
+        else:
             numeric_grads = user_defined_grads or [
                 get_numeric_gradient(
                     numeric_place,
@@ -2421,17 +2432,6 @@ class OpTest(unittest.TestCase):
                 for input_to_check in inputs_to_check
             ]
 
-        if self.is_fp16_compared_with_fp32():
-            self.enable_cal_ref_output()
-            numeric_grads = self._get_gradient(
-                inputs_to_check,
-                place,
-                output_names,
-                no_grad_set,
-                user_defined_grad_outputs,
-            )
-            self.disable_cal_ref_output()
-
         analytic_grads = self._get_gradient(
             inputs_to_check,
             place,