Fix --reduce_memory in finetune_on_pregenerated
On reviewing the code I realized the --reduce_memory code path in `finetune_on_pregenerated.py` had a bug, but also wasn't getting used because the relevant argument wasn't getting passed correctly. The bugs have been fixed and the argument is now passed correctly. Performance still seems good, so now it should be possible to train without loading the whole epoch of training data into memory.
Good catch!