firepony: Fix broken batch size selection
firepony: Fix broken batch size selection

Fixes a serious bug that caused the largest possible batch size to always be
picked, leading to out of memory errors on GPUs with less than 12GB of memory.

broadinstitute/firepony,NVlabs/firepony,broadinstitute/firepony,NVlabs/firepony,broadinstitute/firepony,broadinstitute/firepony,NVlabs/firepony,NVlabs/firepony