seq2seq/finetune.py can take config train_params through command line
- this might belong in lightning_base
- check how/if trainer does it.

Desired API:
```bash
python finetune.py --encoder_layerdrop 0.1 --decoder_layerdrop 0.1 --dropout 0.1 --attention_dropout 0.1
```
@stas00 this might be up your alley.<|||||>Added to my todo queue - thank you, @sshleifer<|||||>https://github.com/huggingface/transformers/pull/6149