[Tasks] Adds image captioning 
This PR adds a guide on image captioning introducing a brand new section for tasks: Multimodal :) 

The idea was originated by @NielsRogge and the PR builds on top of his great work [here](https://github.com/NielsRogge/Transformers-Tutorials/blob/master/GIT/Fine_tune_GIT_on_an_image_captioning_dataset.ipynb). 

An interactive Colab Notebook that was used to develop the task guide is available [here](https://colab.research.google.com/drive/1sLiSY2ixv52yqlw9EUW6BPwCq9XPLc9R?usp=sharing).

Not bad quality, eh?

![image](https://user-images.githubusercontent.com/22957388/217499831-c41ac0f5-2e9c-4b89-aab9-3db96d0066cb.png)

Generated caption: 

> a drawing of a pink and blue pokemon 
_The documentation is not available anymore as the PR was closed or merged._