singa: AssertionError for the ONNX training testcases?

AssertionError with the onnx testcase: https://github.com/apache/singa/blob/master/examples/onnx/training/train.py

$ cd examples/onnx
$ python3 training/train.py --model vgg16

Then I get the following error msg:

File "training/train.py", line 437, in <module>
    args.onnx_model_path, args.data, sgd, args.graph, args.verbosity)
  File "training/train.py", line 295, in run
    model.compile([tx], is_train=True, use_graph=graph, sequential=sequential)
  File "/home/extend/lijiansong/work-space/anaconda2/envs/intel-caffe/lib/python3.6/site-packages/singa/model.py", line 177, in compile
    self.forward(*inputs)
  File "/home/extend/lijiansong/work-space/anaconda2/envs/intel-caffe/lib/python3.6/site-packages/singa/layer.py", line 63, in wrapper
    return func(self, *args, **kwargs)
  File "training/train.py", line 191, in forward
    y = self.linear(y)
  File "/home/extend/lijiansong/work-space/anaconda2/envs/intel-caffe/lib/python3.6/site-packages/singa/layer.py", line 110, in __call__
    return self.forward(*args, **kwargs)
  File "/home/extend/lijiansong/work-space/anaconda2/envs/intel-caffe/lib/python3.6/site-packages/singa/layer.py", line 61, in wrapper
    self.initialize(*args, **kwargs)
  File "/home/extend/lijiansong/work-space/anaconda2/envs/intel-caffe/lib/python3.6/site-packages/singa/layer.py", line 45, in wrapper
    'initialize function expects PlaceHolders or Tensors')
AssertionError: initialize function expects PlaceHolders or Tensors

Something maybe wrong with the layer initialization?

singa version: 3100(the latest build from the source code of master branch) Python version: 3.5.2 ONNX version: 1.5.0