[Generate] Remove attention_mask and integrate model_main_input_name
Author: patrickvonplaten
Pull request: yes

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Final clean-up of generate leveraging the `main_input_name` PR: https://github.com/huggingface/transformers/pull/14803 to:
- Remove `attention_mask` hack for vision models
- clean up the trainer_seq2seq to fix: https://github.com/huggingface/transformers/issues/13825
- clean up hard-coded input names in `generate`

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
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Comments:
The changes to `Seq2SeqTrainer` also fix a problem with `predict_with_generate` for `VisionEncoderDecoder`.

With 4.15.0, setting `predict_with_generate=True`does not work with `VisionEncoderDecoder` and `Seq2SeqTrainer`.


Hey @cgawron,

Do you have an issue with current master or is everything resolved? :-) Happy to make some more changes to make VisionEncoderDecoder work, but if I understand correctly everything is fine now no? 

@patrickvonplaten Yes, current master works fine for me.

@cgawron for me it also works fine with v4.15, not sure what the issue is?