Ability to specify directory of tensorboard logs in BART finetuning example
# 🚀 Feature request

Using [this script](https://github.com/huggingface/transformers/blob/839bfaedb21e42edee093b9e21e2c2f1ea7514f0/examples/summarization/bart/finetune.py) to finetune BART, it'd be useful if we could specify the directory to store the tensorboard logs. Right now, it looks like the logs are saved to `lightning_logs/version_X`. 

Ideally, this is the command I would run:
```{bash}
python finetune.py \
--data_dir=./cnn-dailymail/cnn_dm \
--model_name_or_path=bart-large \
--learning_rate=3e-5 \
--train_batch_size=4 \
--eval_batch_size=4 \
--output_dir=$OUTPUT_DIR \
--do_train  \
--logging_dir logs $@
```

## Motivation

Being able to specify the directory of your tensorboard logs makes it easier to compare multiple experiments.

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