Expand the model zoo (example model set)

SINGA has multiple example models at http://singa.apache.org/docs/examples/
Some are implemented from scratch and some are converted from ONNX, which has a bigger model zoo https://github.com/onnx/models.
The task is to convert more onnx models and implement some popular (and interesting) models that are not in onnx model zoo.
Here are some reference model zoos https://modelzoo.co/, https://gluon-nlp.mxnet.io/model_zoo/index.html