Implemented Coca architecture

[VarunS1997]

What does this PR do?

Implements the work done in the "CoCa": Contrastive Captioners are Image-Text Foundation Models" (https://arxiv.org/pdf/2205.01917.pdf).

This PR requires:

• [x] Model Implementation
• [x] Documentation
• [ ] Testing

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [x] Did you read the contributor guideline, Pull Request section?
• [ ] Was this discussed/approved via a Github issue? Please add a link to it if that's the case.
• [ ] Did you write any new necessary tests?
• [ ] If this adds a new model, can you run a few training steps on TPU in Colab to ensure that no XLA incompatible OP are used?

[divyashreepathihalli]

One additional overhead work is needed.

please add keras_cv/models/feature_extractor/coca to this file https://github.com/keras-team/keras-cv/blob/master/.kokoro/github/ubuntu/gpu/build.sh to line 72 and 86

PS: we will fix this overhead soon, but in the mean time this is what we need to do to make sure the large GPU tests run.

[divyashreepathihalli]

@VarunS1997 will you be completing this one?