Add `FastBert` model

[JulesBelveze]

This PR aims at adding support for the FastBert model.

• [x] add PyTorch model
• [x] add Lightning module
  • [x] main logic
  • [x] metrics computation
  • [x] weights saving & loading
• [x] callback to handle the two steps training procedure

References:

• FastBERT: a Self-distilling BERT with Adaptive Inference Time
• List of existing implementations