[Fix doc example] Wrong checkpoint name
Author: ydshieh
Pull request: yes

# What does this PR do?

In a few model scripts, we have something like (MBart/Blenderbot/Marian)

```
>>> from transformers import BlenderbotTokenizer, BlenderbotForCausalLM

>>> tokenizer = BlenderbotTokenizer.from_pretrained("facebook/bart-large")
>>> model = BlenderbotForCausalLM.from_pretrained("facebook/bart-large", add_cross_attention=False)
```
where `bart-large` was due to `Copied from ...`, and should be changed to the correct checkpoint identifier. (otherwise, in these cases, an error will be thrown if one runs these examples)


## Who can review?

Models:

- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj


Comments:
The failure is coming from the fact there is conflict between style and fix-copies

`style` will change
```
        >>> model = BlenderbotSmallForCausalLM.from_pretrained("facebook/blenderbot_small-90M", add_cross_attention=False)
```
to
```
        >>> model = BlenderbotSmallForCausalLM.from_pretrained(
        ...     "facebook/blenderbot_small-90M", add_cross_attention=False
        ... )
```
but `fix-copies` won't be happy with this change.
cc @sgugger 

> The failure is coming from the fact there is conflict between style and fix-copies
> 
> `style` will change
> 
> ```
>         >>> model = BlenderbotSmallForCausalLM.from_pretrained("facebook/blenderbot_small-90M", add_cross_attention=False)
> ```
> 
> to
> 
> ```
>         >>> model = BlenderbotSmallForCausalLM.from_pretrained(
>         ...     "facebook/blenderbot_small-90M", add_cross_attention=False
>         ... )
> ```
> 
> but `fix-copies` won't be happy with this change. cc @sgugger

Maybe store the docstring in a variable `CAUSAL_LM_EXAMPLE` and then use it with `add_end_docstrings` like we do for generation example in `BartForConditionalGeneration`

The # Copied from should be removed from the two models that need to have their code examples adapted: the change of checkpoint makes them go over the 119 char limit so `style_doc` wants to style them (using black).

> The # Copied from should be removed from the two models that need to have their code examples adapted: the change of checkpoint makes them go over the 119 char limit so `style_doc` wants to style them (using black).

I am OK to remove `# Copied` - but let's see what @patrickvonplaten says

IMO we should store the example `docstr` in a variable and add it to doc using the `add_end_docstrings` annotation for all these classes. 
Since these are all the same models having  `copied from...` would be better.

I think it is good if we can keep `copy from` as many as possible - if there is a way to do it.
Unless there is other opinions or workarounds, I will try @patil-suraj 's suggestion.

+1 on @patil-suraj's suggestion

If you wait a bit, I'll have a fix on the copies script today or tomorrow (they'll apply black to the examples as well so that this kind of problem disappears).

> If you wait a bit, I'll have a fix on the copies script today or tomorrow (they'll apply black to the examples as well so that this kind of problem disappears).

Sure, thanks for the information.

If you rebase to include the PR mentioned above, run `make fix-copies`, the error should disappear.

@sgugger Thanks, it works well!

@patil-suraj Fix for Pegasus done as you pointed out.

Thanks a lot!