Add torch-kbnufft

[paquiteau]

torch-kbnufft is used in the DL community. Yet better nufft libraries are available (e.g. cufinufft, gpunufft). The idea would be add torch-kbnufft in order to the run the benchmark, and show what is the best nufft currently.

Note that torch-kbnufft has also some interesting capabilities, notably differentiation and support for toeplitz embedding (speeds things up in data consistency evaluation)