concat_title_abstract must be True in scripts/pytorch_lightning_training_script/train.py#L73

Hi @armancohan.

In [scripts/pytorch_lightning_training_script/train.py#L73](https://github.com/allenai/specter/blob/master/scripts/pytorch_lightning_training_script/train.py#L73), `concat_title_abstract` must be `True` if you want to train both with `title` and `abstract`.

However, it is setup to `None` in the actual script. That means that only the document `title` is used for training.

`concat_title_abstract: bool = None` # actual code

should be:

`concat_title_abstract: bool = True`

What do you think? Thank you.