Add DFFT

[NielsRogge]

Model description

DFFT is a new fully Transformer-based object detector. The model doesn't require a decoder, unlike DETR.

Open source status

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

Provide useful links for the implementation

Paper: https://arxiv.org/abs/2206.06829

Github repo (and weights): https://github.com/Pealing/DFFT