finetune_on_pregenerate Loss.backwards() throw an error 
In finetune_on_pregenerated.py, loss are tuples and thus loss.backward() is not going to work.

Original:
loss = model(input_ids, segment_ids, input_mask, lm_label_ids, is_next)

Update:
loss, _ , _ = model(input_ids, segment_ids, input_mask, lm_label_ids, is_next)

Is this fix correct?
Yes this example should have been updated now with #797.