Online DPO scheduler step before optimizer step

[edbeeching]

I see this warning in the test logs when testing online DPO:

tests/test_online_dpo_trainer.py::TestOnlineDPOTrainer::test_online_dpo_trainer_training
  C:\hostedtoolcache\windows\Python\3.10.11\x64\lib\site-packages\torch\optim\lr_scheduler.py:216: UserWarning: Detected call of `lr_scheduler.step()` before `optimizer.step()`. In PyTorch 1.1.0 and later, you should call them in the opposite order: `optimizer.step()` before `lr_scheduler.step()`.  Failure to do this will result in PyTorch skipping the first value of the learning rate schedule. See more details at https://pytorch.org/docs/stable/optim.html#how-to-adjust-learning-rate

So it seems the opt.step and scheduler.step are called in the wrong order.

[qgallouedec]

Fixed in #1918