Additional supports for autodiff

[chaithyagr]

We soon will support autodiff wrt to trajectory. However, we should perhaps handle:

• [ ] autodiff wrt to smaps
• [x] https://github.com/mind-inria/mri-nufft/pull/116#discussion_r1626161177

We need the users to be able to use autodiff wrt to trajectory more directly as:

trajectory = torch.Tensor(...) trajectory.requires_grad = True

get_operator('cufinufft')(trajectory)

Note that if trajectory.requires_grad = True, we can enable grad_wrt_traj as True internally. We dont want users to write it twice.