Add FlexiBERT
### Model description

FlexiBERT is a suite of *flexible* and *heterogeneous* models. The design space was proposed in this [paper](https://arxiv.org/abs/2205.11656) accepted for publication in the Journal of Artificial Intelligence Research.

### Open source status

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

### Provide useful links for the implementation

The weights are available [here](https://huggingface.co/shikhartuli/flexibert-mini).
The PR was closed by bot. Please reopen the PR and let me know if anything needs to be done to merge the PR. The PR would add the FlexiBERT suite of models to 🤗 Transformers.