[Fix doc example] Wrong checkpoint name
# 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

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!