Add Flax implementation for BLOOM

### Model description

I'm interested in adding an implementation of BLOOM in Flax.

The implementation shouldn't be too bad since the pytorch implementation can serve as a guide and a way to check correctness.

### Open source status

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

### Provide useful links for the implementation

@younesbelkada @stas00 @patrickvonplaten

If someone is already planning to work on this then no worries, but if not I will start on this as soon as I have time!

Start by reading the existing PyTorch BLOOM implementation and the repository's Flax model implementations for structure and conventions. Use the PyTorch version as the guide and correctness reference; done means a working Flax BLOOM implementation with available weights and behavior matching the reference.

Written by the indexing model from the issue text.