fix the description of token used for Bart classification
Author: ekagra-ranjan
Pull request: yes

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Fixes the description of token used for Bart classification.  It uses last EOS token and not a special pooled output.

https://github.com/huggingface/transformers/blob/7a8118947f3c6a802a9f63dc22c394961d38860f/src/transformers/models/bart/modeling_bart.py#L1520-L1527

## Before submitting
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@patrickvonplaten, @patil-suraj

Comments:
_The documentation is not available anymore as the PR was closed or merged._

@patrickvonplaten - does this look fine to you?

@ArthurZucker @gante could you take a look here? 

> @ArthurZucker @JoaoLages could you take a look here?

Maybe you wanted to call João Gante and not me, but here are my 2 cents anyway 😄 : the description that @ekagra-ranjan wrote is correct but the previous description was not incorrect either. 
Taking the last EOS token embedding from the model's output is a type of pooling. For example, [in BERT we take the first token instead, that corresponds to the CLS token](https://github.com/huggingface/transformers/blob/7a8118947f3c6a802a9f63dc22c394961d38860f/src/transformers/models/bert/modeling_bert.py#L653)  and we also have [the same description for `BertForSequenceClassification`](https://github.com/huggingface/transformers/blob/7a8118947f3c6a802a9f63dc22c394961d38860f/src/transformers/models/bert/modeling_bert.py#L1509). 
The previous description is simpler, more general and it is not incorrect. Not against having more descriptive docstrings, but then it would make sense to review all the `(...)ForSequenceClassification` classes, not only BART.


Sorry @JoaoLages :sweat_smile: ! You are right indeed

@JoaoLages I see your point and I guess you are right! Thanks for sharing your thoughts.