[cherry-pick] set int64 for Output(Length) of sequence_pad, test=develop (#24161) (#24658)

Author: Zhang Ting

python/paddle/fluid/layers/sequence_lod.py

@@ -18,6 +18,7 @@
 from ..framework import Variable, in_dygraph_mode
 from ..layer_helper import LayerHelper
 from ..data_feeder import check_variable_and_dtype, check_type, check_dtype
+from ..core import VarDesc
 
 __all__ = [
     'sequence_conv',
@@ -963,7 +964,7 @@ def sequence_pad(x, pad_value, maxlen=None, name=None):
                              'fluid.layers.sequence_pad')
     dtype = helper.input_dtype()
     out = helper.create_variable_for_type_inference(dtype)
-    length = helper.create_variable_for_type_inference(dtype)
+    length = helper.create_variable_for_type_inference(VarDesc.VarType.INT64)
 
     pad_value.stop_gradient = True
     length.stop_gradient = True

python/paddle/fluid/tests/unittests/sequence/test_sequence_pad_op.py

@@ -19,6 +19,7 @@
 from op_test import OpTest
 
 import paddle.fluid as fluid
+import paddle.fluid.core as core
 
 
 class TestSequencePadOp(OpTest):
@@ -173,6 +174,13 @@ def test_dtype():
 
         self.assertRaises(TypeError, test_dtype)
 
+    def test_length_dtype(self):
+        x = fluid.data(name='x', shape=[10, 5], dtype='float32', lod_level=1)
+        pad_value = fluid.layers.assign(input=np.array([0.0], dtype=np.float32))
+        out, length = fluid.layers.sequence_pad(x=x, pad_value=pad_value)
+        # check if the dtype of length is int64 in compile time
+        self.assertEqual(length.dtype, core.VarDesc.VarType.INT64)
+
 
 if __name__ == '__main__':
     unittest.main()