Your input and target tensors are DoubleTensors, but your model parameters are FloatTensors. You have to convert either your inputs or your parameters.
To convert your inputs to float (recommended):
inputs, labels = data
inputs = inputs.float()
labels = labels.float()
inputs, labels = Variable(inputs), Variable(labels)

To convert your model to double:
model = ColorizerNet()
model.double()

I recommend using floats instead of doubles. It’s the default tensor type in PyTorch. On GPUs, float calculations are much faster than double calculations.