Training of machine translation model fails if config parameter `trainer.max_epochs` is used instead of `trainer.max_steps`.
Replace `trainer.distributed_backend` with `trainer.accelerator` everywhere in the repo, because `distributed_backend` is [renamed ](https://pytorch-lightning.readthedocs.io/en/latest/generated/CHANGELOG.html?highlight=distributed_backend#id119) into `accelerator` in `pytorch-lightning==1.0.5`.

Closes #3104 