Small QOL improvements to TrainingArguments
# What does this PR do?

Some small QOL improvements as discussed on the [forum](https://discuss.huggingface.co/t/seq2seqtrainer-questions/1276/):
- make `do_eval` defaults to `evaluation_strategy != "no"` so there is no need to pass the two
- make `run_name` defaults to `output_dir`.
Thanks!