If your finetuned model is just tuned (and not extended with extra layers) then save_pretrained and from_pretrained should work, and may be the easiest.
If you have altered your model (with extra layers) then I think you need to use torch.save and torch.load, and redefine your model configuration before loading, eg
torch.save(model.state_dict(),‘filepath’)
model = ATBertClass()                      # redefine using your custom model definition
model.load_state_dict(torch.load(‘filepath’))