get rid from bart attention copypaste
Author: kventinel
Pull request: yes

# What does this PR do?

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Fixes # (issue)


## Before submitting
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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19642). All of your documentation changes will be reflected on that endpoint.

Thanks for your PR, but having the models be independent of each other is a core principle of the philosophy of the library. You can learn more about it in [this blog post](https://huggingface.co/blog/transformers-design-philosophy).

> Thanks for your PR, but having the models be independent of each other is a core principle of the philosophy of the library. You can learn more about it in [this blog post](https://huggingface.co/blog/transformers-design-philosophy).

Even small functions like `expand_mask`?

Also in some files this PR catch problem, when some functions/classes was copied without `#Copied from` mechanism. 

P.S. In some way this PR not very different from your ~D~RY philosophy, because arguments like easy to patch one files stiil exists, because each one can change Attention for own file like he wants using just copy and change.