fix sequence_pad example error

test=develop

python/paddle/fluid/layers/nn.py

@@ -3016,7 +3016,8 @@ def sequence_pad(x, pad_value, maxlen=None, name=None):
 
             x = fluid.layers.data(name='y', shape=[10, 5],
                              dtype='float32', lod_level=1)
-            pad_value = fluid.layers.assign(input=numpy.array([0]))
+            pad_value = fluid.layers.assign(
+                input=numpy.array([0], dtype=numpy.float32))
             out = fluid.layers.sequence_pad(x=x, pad_value=pad_value)
     """