Add new operators and layers

[nudles]

SINGA has many common operators and layers. There are also many operators to be implemented. Here is a list of popular operators define by ONNX https://github.com/onnx/onnx/blob/master/docs/Operators.md Here is the list of operators implemented in SINGA http://singa.apache.org/docs/onnx/#supported-operators The task is to add the operators in ONNX but not in SINGA. Refer to this link for how to add new operators and corresponding layers into SINGA.