Pixel batcher

[jpuigcerver]

It would be very interesting to have a Pixel batcher, so that instead of sampling a fixed number of images, to sample an upper bound of pixels. This way, we could make a more efficient use of the GPU memory, and the batch size would not be affected by the largest image size, as it is affected now (batches with bigger images would contain less samples).

Aachen's RETURNN supports this options and I think it's very interesting.

[jpuigcerver]

On a second thought, it would be even better if the user just specified the number of MB. That way the memory required would only depend on the model.