Support for `Flax` Trainer
### Feature request

A `Trainer` class similar to PyTorch/Tensorflow (deprecating in v5) . 

### Motivation

The process of training HuggingFace models in `PyTorch` has become more accessible due to the availability of the `Trainer` class and extensive documentation and tutorials. Similarly, in `Tensorflow`, training models require only a single line of code (`model.fit`). and it makes sense to deprecate the current trainer to avoid redundancy. 


However, the process of training `Flax` models currently requires boilerplate code similar to `PyTorch` which a `Trainer` class would be helpful to eliminate. Making flax `Trainer` available will allow for really fast training in GPU/TPUs and with the support of weights conversion, one can instantly convert the model to `Tensorflow`/`PyTorch` for inference and deployment.

### Your contribution

I can submit the PR. 

cc @sanchit-gandhi since it's related to flax. 
This is also something we do not want to add or maintain for Flax, as researchers usually dislike Trainer classes very much.<|||||>Understood, thanks for such a quick reply :hugs: 