Added SegFormer3D arch compatible with HF Transformers structure

[bnavard]

1. Current modeling impl is compatible with HF format. The goal is to integrate our model to Transformers open model repo
2. Added config script to be imported to the main model parameters for better scalability
3. SegFormer3D handles all the shapes bug free. No need to be cube root shape
4. Reformatted the code-base with black
5. updated .gitignore by removing unnecessary lines.

TODO:

• [ ] check if the output shape is expected behavior
• [ ] check for any logical bug withing our modeling_segformer3d.py script
• [ ] train the supposed bug free model on all the data sets used in our paper