fix vision enc-dec models conversion to onnx
Author: kventinel
Pull request: yes

# What does this PR do?

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Fixes #19811


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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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- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19819). All of your documentation changes will be reflected on that endpoint.

cc @lewtun 

Hello @kventinel ,  thanks for the PR. I am currently looking for the same thing for Whisper model and was going to update the VisionEncoderDecoderModel soon. The better way would be to run the full model by passing in the `encoder_outputs`. It gives you decoder + other parts but skips the encoder. Hence, it would be more generic than creating separate functions.

You can follow up on the PR's [19525](https://github.com/huggingface/transformers/pull/19525) and [420](https://github.com/huggingface/optimum/pull/420/files#diff-c27ea812737bc6ccfe34f92a4ff0d1ec473a41b8c8012bfdb08bb22a46104ddeR321) for the changes for adding the encoder_outputs. You could add the encoder_outputs export for the model in a similar manner after the PR 19525 is merged.

Hi @kventinel , since the PR [19525](https://github.com/huggingface/transformers/pull/19525) is merged would you like to update the model config to use `encoder_outputs`?  Let me know if you have any questions.

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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