Simpletransformers

Added

- Added LayoutLM for Classification.
- Added MBart. [Zhylkaaa](https://github.com/Zhylkaaa)
- Added BERTweet for NER. [manueltonneau](https://github.com/manueltonneau)
- Added Longformer for Multilabel Classification. [manueltonneau](https://github.com/manueltonneau)

Fixed

- Fixed issue with `Seq2SeqModel` when the `model_name` contained backslashes.
- Fixed issue with saving args when a `dataset_class` is specified in `Seq2SeqModel`.

Changed

- The Electra implementation used with `ClassificationModel` is now fully compatible with Hugging Face.

Fixed

- Updated some tokenizer arguments to the new argument names. [macabdul9](https://github.com/macabdul9)
- Learning rate is now obtained from the `get_last_lr()` method. [sarapapi](https://github.com/sarapapi)