Bug fix: exporting Alexnet and VGG models to ONNX

ONNX export in PyTorch doesn't know how to handle DataParallel
layers, so we need to make sure that we remove all instances
of nn.DataParallel from the model before exporting it.

The previous ONNX implementation forgot to deal with the case
of DataParallel layers that do not wrap the entire model (as
in VGG, where only the feature-extractor layers are data-parallel).