[Bert2Bert] allow bert2bert + relative embeddings
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Fixes #14010
Everything is explained in #14010. IMO, Bert2Bert like models should not (and also cannot really) make use of positional bias in the cross_attention_layers. The PR forces cross attention layers to always use `"absolute"` position encodings.

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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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HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
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Comments:
Thanks @patrickvonplaten for fixing this! It looks correct that relative position embeddings should not be used on cross-attention layers.

Merging now to prevent merge conflicts