`facebook/bart-large-mnli` - random accuracy on MNLI
# 🐛 Bug

## Information

Model I am using (Bert, XLNet ...): BART

Language I am using the model on (English, Chinese ...): English

The problem arises when using:
* [X] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [X] an official GLUE/SQUaD task: MNLI
* [ ] my own task or dataset: (give details below)

## To reproduce

There might be something wrong with the `facebook/bart-large-mnli` checkpoint (https://huggingface.co/facebook/bart-large-mnli). I can only get a random accuracy (0.35) on MNLI:
`python examples/text-classification/run_glue.py --data_dir <GLUE DATA DIR/MNLI> --task MNLI --model_name_or_path facebook/bart-large-mnli --output_dir ./dbg/ --max_seq_length 128 --overwrite_cache --do_eval`

I checked and the checkpoint has a classification head (`'classification_head.dense.bias', 'classification_head.out_proj.weight', 'classification_head.out_proj.bias'`) i.e. the classification head is not initialized randomly.

cc @sshleifer as discussed