Imagenet: change color channel implementation

[laurasootes]

At the moment there are a number of hardcoded things for the imagenet model in the dashboard.

One of them is related to the fact that the Resnet model requires the image to be in 244*244 size with the color channel in position 0, and to normalise the image. This is now implemented in dashboard utilities, but requires some hacky way to determine that you are trying to do the ResNet model, which is not ideal.