Enable batch-parallelization on multiple GPUs

[matt-gardner]

This should be doable with Keras, as shown here: https://medium.com/@kuza55/transparent-multi-gpu-training-on-tensorflow-with-keras-8b0016fd9012. Matt Peters also got this working, though I'm not sure if he used the approach in that blog post, and he had to fork Keras to get it to work correctly.

[matt-gardner]

#326