Add DFFT
### 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
Hello, is this issue still open? I would like to contribute to it.<|||||>@NielsRogge Anyone working on this?

Thank you<|||||>Feel free to start working on this!<|||||>@NielsRogge if no one is working on this then I would like to start working on it<|||||>Cool, feel free to open a draft PR<|||||>Hi @NielsRogge, @soma2000-lang, are there any updates on this issue? I would like to contribute.<|||||>@atharvakavitkar  working to open a draft pr