[PYTHON] Clear memory right at the end of destruction

[chaithyagr]

The current version of codes calls delete for gpuNUFFTOp on destruct.
However, I see that the GPU Memory is not cleared.
@andyschwarzl Do you have any idea what I could have missed and are we sure that there arent any memory leaks in the base set of codes?

[andyschwarzl]

The base set of the code should not lead to any GPU memory leaks, but what I've seen was memory fragmentation when working with the same device context and instantiating the gpuNUFFT operator several times.

[chaithyagr]

Hmmm, interesting.
This seems to be a specific issue with python codes in that case. I have a feeling this comes up if the NUFFTOp object is not destroyed. Also, this could be due to the fact that memory ownership could be passed to python. I will look into this and try to get a PR