[s2s] add config params like Dropout in Seq2SeqTrainingArguments
# What does this PR do?

1. Adds `config` params (`encoder_layerdrop`, `decoder_layerdrop`, `dropout`, `attention_dropout`) in `Seq2SeqTrainingArguments`
2. Fix T5 warnings (don't pass src_lang, tgt_lang args to `T5Tokenizer`)
3. Correct `vocab_size` for `FSMT`.
4. Fix `test_finetune_trainer_slow`
5. minor code cleanup in `Seq2SeqTrainer`

@sshleifer 
Yes, will add lr scheduler in separate PR <|||||>@sshleifer  anything missing in this PR ?