DirectoryParsingLabelDataset fails to read images with an alpha channel

[yoshnary]

When I tried the ImageNet training example, it failed due to DirectoryParsingLabelDataset. ImageNet 2012 dataset has an image with an alpha channel, and DirectoryParsingLabelDataset cannot read it. I dealt with it by adding alpha='ignore' to the following function call.

chainercv/chainercv/datasets/directory_parsing_label_dataset.py

Line 148 in ca45a7f

return read_image(self.img_paths[i], color=self.color)

Could you fix it in a proper way?