Provide Benchmark Backbone Checkpoint Weights

[guarin]

Right now we only upload checkpoint weights for the full SSL models. But those can be a bit hard to work with as you have to modify fields in the state dict if you only want to load the backbone. We should upload the weights for the backbone only to make them more easily usable.

Tasks

• Save backbone state dict after pretraining (here). This can be done with torch.save(model.backbone.state_dict(), path). Path should be a path in the same directory as the full checkpoint. Do the same for vit benchmarks.
• Save backbone state dict for existing pretrained models and upload them to S3. This can be done with

checkpoint = torch.hub.load_state_dict_from_url(url, map_location="cpu")

# Remove 'backbone.' prefix from keys in state_dict
backbone_state_dict = {
    k.replace("backbone.", ""): v
    for k, v in checkpoint["state_dict"].items()
    if k.startswith("backbone.")
}
torch.save(backbone_state_dict, path)

• Add backbone checkpoint links in readme and docs
• Update the tutorial to use these weights: https://docs.lightly.ai/self-supervised-learning/tutorials/package/tutorial_checkpoint_finetuning.html