Add BridgeTowerForContrastiveLearning
Author: abhiwand
Pull request: yes

# What does this PR do?

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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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## Who can review?

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Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @sgugger

Integrations:

- deepspeed: HF Trainer: @stas00, Accelerate: @pacman100
- ray/raytune: @richardliaw, @amogkam

Documentation: @sgugger, @stevhliu and @MKhalusova

HF projects:

- accelerate: [different repo](https://github.com/huggingface/accelerate)
- datasets: [different repo](https://github.com/huggingface/datasets)
- diffusers: [different repo](https://github.com/huggingface/diffusers)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Maintained examples (not research project or legacy):

- Flax: @sanchit-gandhi
- PyTorch: @sgugger
- TensorFlow: @Rocketknight1

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

@sgugger We have addressed all of your review in the latest commit. We have also added tests for BridgeTowerForContrastiveLearning. Would you please review the latest commit? We are looking forward to having this PR merged into main.
Thanks

Thanks for adding this model @abhiwand @tileintel !  Great to see it merged into the library :) 

For the model tests, some of the configuration values in `BridgeTowerModelTester` result in large models being created and used in the test suite e.g. `vocab_size = 50265` set [here](https://github.com/huggingface/transformers/blob/bcc8d30affba29c594320fc80e4a4422fb850175/tests/models/bridgetower/test_modeling_bridgetower.py#L97), which results in periodic OOM errors in the CI runs. 

Could you add a follow up PR for `BridgeTowerModelTester` and `BridgeTowerModelTest` to have smaller default values to create lighter tests?  A good reference for this would be [CLIP](https://github.com/huggingface/transformers/blob/main/tests/models/clip/test_modeling_clip.py). Ideally we would also have a similar structure of test classes for the different modalities i.e. `BridgeTower[Text|Vision]ModelTest(er)`.