Make learning rate rescale based on mini-batch size rather than # workers

[kyledmiller]

Currently, mala rescales learning rate based on # of workers which indirectly follows the linear scaling rule (Goyal et al.) if mini batch size is variably calculated based on # of workers. But this isn't always the case as mini batch size can be fixed separately. I think we should scale LR based on the mini batch size instead to ensure we are following the linear scaling rule.