fix Flatten api test=develop (#26346)

python/paddle/fluid/dygraph/nn.py

@@ -14,6 +14,7 @@
 
 from __future__ import print_function
 
+import paddle
 from six.moves import reduce
 from .. import core
 from ..layers import utils
@@ -3457,19 +3458,6 @@ class Flatten(layers.Layer):
         self.stop_axis = stop_axis
 
     def forward(self, input):
-        out = self._helper.create_variable_for_type_inference(input.dtype)
-        x_shape = self._helper.create_variable_for_type_inference(input.dtype)
-
-        if in_dygraph_mode():
-            dy_out, _ = core.ops.flatten_contiguous_range(
-                input, 'start_axis', self.start_axis, 'stop_axis',
-                self.stop_axis)
-            return dy_out
-        self._helper.append_op(
-            type="flatten_contiguous_range",
-            inputs={"X": input},
-            outputs={"Out": out,
-                     "XShape": x_shape},
-            attrs={"start_axis": self.start_axis,
-                   "stop_axis": self.stop_axis})
+        out = paddle.tensor.manipulation.flatten(
+            input, start_axis=self.start_axis, stop_axis=self.stop_axis)
         return out