Create TF port of BigBird
### Model description

[BigBird](https://arxiv.org/abs/2007.14062) is an open source transformer model architecture for longer sequences, and is implemented in the Transformer library already in PyTorch and Flax, but not yet in TensorFlow. This issue tracks the implementation of a TensorFlow version of the model.

### Open source status

- [X] The model implementation is available
- [X] The model weights are available

### Provide useful links for the implementation

[Location of current implementations](https://github.com/huggingface/transformers/tree/main/src/transformers/models/big_bird)
Currently starting to work on this :)