use core.op in add_position_encoding API.

python/paddle/fluid/layers/nn.py

@@ -13166,6 +13166,10 @@ def add_position_encoding(input, alpha, beta, name=None):
                 input=tensor, alpha=1.0, beta=1.0)
 
     """
+    if in_dygraph_mode():
+        return core.ops.add_position_encoding(input, "alpha", alpha, "beta",
+                                              beta)
+
     helper = LayerHelper('add_position_encoding', **locals())
     check_variable_and_dtype(input, 'input', ['float32', 'float64'],
                              "add_position_encoding")