I verified the below two things -
for p in model.backbone.parameters():
    assert p.grad==None


forzen_backbone = timm.create_model('ssl_resnet50', True, num_classes=0).eval()
# model.backbone was also frozen during training
for p, p2 in zip(model.backbone.parameters(), forzen_backbone.parameters()):
    print(torch.all(torch.isclose(p, p2)))


Prints all True