pre padding in dygraph (#30179)

Change-Id: Ia5279b0cbb6a5b3970aff66e9510e0d85efa70ce

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

@@ -26,12 +26,10 @@ paddle.disable_static()
 class EmbeddingDygraph(unittest.TestCase):
     def test_1(self):
         x_data = np.arange(3, 6).reshape((3, 1)).astype(np.int64)
-        y_data = np.arange(6, 12).reshape((3, 2)).astype(np.float32)
         paddle.disable_static(paddle.CPUPlace())
         x = paddle.to_tensor(x_data, stop_gradient=False)
-        y = paddle.to_tensor(y_data, stop_gradient=False)
 
-        embedding = paddle.nn.Embedding(10, 3, sparse=True)
+        embedding = paddle.nn.Embedding(10, 3, sparse=True, padding_idx=9)
 
         w0 = np.full(shape=(10, 3), fill_value=2).astype(np.float32)
         embedding.weight.set_value(w0)

python/paddle/nn/layer/common.py

@@ -16,6 +16,7 @@
 import paddle
 from ...fluid.dygraph import Flatten  #DEFINE_ALIAS
 from ...fluid.dygraph import layers
+from ...fluid.framework import in_dygraph_mode
 from .. import functional as F
 from ...fluid.framework import _dygraph_tracer
 
@@ -1277,6 +1278,9 @@ class Embedding(layers.Layer):
             dtype=self._dtype,
             is_bias=False)
 
+        if in_dygraph_mode() and padding_idx != -1:
+            self.weight[padding_idx] = 0.0
+
     def forward(self, x):
         return F.embedding(
             x,