simplify dpt copying
Author: kventinel
Pull request: yes

# What does this PR do?

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
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- gpt2: @patrickvonplaten, @LysandreJik
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Library:

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- tokenizers: @n1t0, @LysandreJik
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- datasets: [different repo](https://github.com/huggingface/datasets)
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Comments:
_The documentation is not available anymore as the PR was closed or merged._

> Thanks for your PR, but we don't accept renaming layers like this as it's a breaking change.

Why? It's even not change `DPTPreTrainedModel`, so probabiity that some one use inner layers not so big.