pointer to transformer (big) model

# 🌟 New model addition

## Model description

Hi, needed a pointer on how to instantiate a Transformer-big from the original Vaswani et. al. paper (Attention Is All You Need). I could only find versions of Transformer-like architectures, so would be useful if this could also be added.

## Open source status

* [x] the model implementation is available: (give details): https://research.google/pubs/pub46201/
* [ ] the model weights are available: (give details)
* [ ] who are the authors: (mention them, if possible by @gh-username)

Start by reviewing the linked Vaswani et al. paper and comparing its Transformer-big architecture with the existing Transformer-like implementations in the repository. Done means adding a usable Transformer-big model implementation with a clear instantiation path; the issue does not name specific files or tests.

Written by the indexing model from the issue text.