Add python API for sum op.

python/paddle/fluid/layers/ops.py

@@ -51,7 +51,7 @@ __all__ = [
     'clip_by_norm', 'softmax', 'sequence_softmax', 'logical_and', 'logical_or',
     'logical_xor', 'logical_not', 'uniform_random',
     'uniform_random_batch_size_like', 'gaussian_random',
-    'gaussian_random_batch_size_like', 'cumsum', 'scatter'
+    'gaussian_random_batch_size_like', 'cumsum', 'scatter', 'sum'
 ] + __activations__
 
 for _OP in set(__all__):