Add hf_hub_download

[NielsRogge]

This PR proposes to make the weights of RF-DETR available at https://huggingface.co/Roboflow.

The download_pretrain_weights method is updated correspondingly to download the weights from the 🤗 hub.

This comes with a few benefits:

• the models are easier discoverable for people, as we can add the "object detection" tag, such that people find them at https://huggingface.co/models?pipeline_tag=object-detection&sort=trending. RF-DETR could become one of the top trending models!
• download stats will be provided, such that you can track usage
• we can add descriptive model cards for better documentation.

Additionally, we could optionally leverage safetensors for a safer format for serializing the weights.

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