bart-base config.*attention_heads (should be 12 was 16)
[Bart-base configuration](https://s3.amazonaws.com/models.huggingface.co/bert/facebook/bart-base/config.json) has the wrong values for `decoder_attention_heads` and `encoder_attention_heads`. This messes up the self-attention computation and make model totally unusable.  

### Who can help
@sshleifer

thx!<|||||>Should it be 12?<|||||>yes<|||||>Fixed, great catch, sorry for the annoyance.<|||||>cc @VictorSanh if you are still using.