A simple example to showcase autodiff

[chaithyagr]

We could have possibly the following examples:

• [ ] A simple U-Net (pulled from fastMRI dataset possibly), trained? @Lenoush can you handle this in your free time?
• [ ] A network built with DeepInv library for some already preset reconstruction (PnP possibly?) @matthieutrs can you handle this this in your free time? I think you can also have something basic passed to @Lenoush and she can work on it forward.
• [x] A simple example where we could learn the trajectory possibly? @alineyyy can you handle this in your free time? #167
• [x] Add additional support for learning a sampling pattern in 2D with straight line readouts. Perhaps we can do it while supporting stackNUFFT on torch? #181