Python implementation refactor, numba kernel optimizations, some GPU implementations

[din14970]

After more than a year I finally found a bit of spare time to work on this project a bit more. I have taken your implementation @berkels and made some improvements in terms of:

• general pythonic code quality & formatting, descriptive naming, separation into modules/classes, use of type hints, ...
• improved performance on the CPU by optimizing numba jit functions, utilization of multiple cores in some of the kernels, and caching of some pre-computed results so they don't get calculated again.
• support for running the calculation on the GPU using cupy to replace numpy and numba.cuda kernels to replace numba.jit (see the cuda_kernels module). Unfortunately at this point it seems GPU performs worse in most cases due to data transfers and allocations.
• unit tests that show the code still works the same way as before and as expected.

What still needs to happen:

• further work out the user facing API that currently resides in implementation/implementation.py, in the JNNR class, support writing intermediate results to a file.
• human readable docs would be nice that explain how the code actually works and implements what is written in the papers. This will not be in the scope of this PR though.
• I don't think the multistage optimization works as intended yet, as described in https://arxiv.org/pdf/1901.01709.pdf. Particularly, I'm not sure how the bias correction step is supposed to be implemented. This is in JNNR.run. Some help here would be appreciated.
• benchmarking against matchseries.

@berkels I would really appreciate if you could skim over this if/when you find time and comment on:

• the bias reduction step issue
• naming of functions and variables, descriptions in docstrings, ... (is it accurate/correct?)
• tips on further improving performance
• anything else you notice

[berkels]

The bias correction step indeed still seems to be missing. Let $$N[\psi]:=\frac12\sum{i=1}^{n}\int\Omega\lVert\phi_i(\psi(x))-x\rVert^2\mathrm{d}x.$$ To compute the minimizing $\phi$, we do a gradient flow of $N$ with respect to $\psi$. This needs the first variation $$\langle N'[\psi,\phi_1,\ldots,\phin],\zeta\rangle=\sum{i=1}^{n}\int_\Omega (D\phi_i)(\psi(x))^T(\phi_i(\psi(x))-x)\cdot\zeta\mathrm{d}x.$$ This is very similar to how the registration itself is done. This just doesn't have a regularizer and includes all deformations, not just one.

[berkels]

Note $N$ can be seen as sum of data terms typical for registration. Let $x_1$ and $x_2$ denote the components of $x$, i.e. $x=(x_1,x_2)$. Furthermore, denote the components of $\phi_i(x)$ with $\phii(x)=(\phi{i,1}(x),\phi{i,2}(x))$. Then, $$N[\psi]=\frac12\sum{i=1}^{n}\sum{j=1}^2\int\Omega(\phi_{i,j}(\psi(x))-x_j)^2\mathrm{d}x.$$ and $$\langle N'[\psi,\phi_1,\ldots,\phin],\zeta\rangle=\sum{i=1}^{n}\sum{j=1}^2\int\Omega(\phi_{i,j}(\psi(x))-xj)\nabla\phi{i,j}(\psi(x)) \cdot\zeta.$$ Using this, you should be able to implement this using the data term and its gradient you implemented for the registration. There the images would be $\phi_{i,j}$ and $x_j$. The C++ code uses the interpretation above, but reusing your existing registration should work as well and take less effort.

[berkels]

BTW: I'm very happy to see that you made a lot of progress on this!

[berkels]

FYI, the C++ code handles the bias correction in SeriesMatching::reduceDeformations in projects/electronMicroscopy/matchSeries.h. I don't think that though this is very helpful as reference.