Add vision_encoder_decoder to models/__init__.py
# What does this PR do?

The recently added `vision_encoder_decoder` wasn't added to `models/__init__.py`. This had some consequence, like `utils.check_repo.get_model_modules()` missing `vision_encoder_decoder`, and therefore models like `VisionEncoderDecoderModel` (and the upcoming Flax/TF versions) would pass `check_all_models_are_tested` even without providing a test file.

This (one-line) PR fixes this issue.

## Who can review?

@sgugger @NielsRogge 
Yes. It's easy. I can fix it in the same PR? It's ok?<|||||>Yes we can't merge it otherwise ;-)<|||||>Done.

P.S.: I don't know the exact reason for which we put encoder-decoder model family in the list `_ignore_modules`, but I saw`modeling_encoder_decoder` was already there since the 1st version `6ba540b7` of `check_repo.py`.

https://github.com/huggingface/transformers/blob/6ba540b7475f095d591b4766cac897007b1d5db0/utils/check_repo.py#L108-L112

The error reported is
```
test_modeling_vision_encoder_decoder.py should define `all_model_classes` to apply common tests to the models it tests. If this intentional, add the test filename to `TEST_FILES_WITH_NO_COMMON_TESTS` in the file `utils/check_repo.py`.
```
I think `TEST_FILES_WITH_NO_COMMON_TESTS` is the place to address (?), but maybe I don't see the full picture here.<|||||>No that's the right way.