The stored checkpoints contains most likely the state_dicts of the model and optimizer.
You would have to create a model instance first and load the state_dict afterwards as explained in the serialization docs 101:
model = MyModel()
optimizer = torch.optim.Adam(model.parameters(), lr=1e-3) # change to whatever optimizer was used

checkpoint = torch.load(path_to_checkpoint)
model.load_state_dict(checkpoint['model'])
optimizer.load_state_dict(checkpoint['opt'])

Note that you need the model definition to create the model instance.