Add Sor kernels

[MarcelKoch]

This PR adds the device kernels for #1633.

[yhmtsai]

it does not help anything below, so I would rather remove it.

[MarcelKoch]

To make this more reasonable, I also removed the factorization namespace in the core file. Otherwise I would have had to use ::gko::factorization:: everywhere instead of just helpers::.

[yhmtsai]

sorry, I thought they are in the same namespace, but one is under kernel namespace and the other is under gko directly.
then the original one is better as it does not give any ambiguous function.
although it can only declare the two class you need, it's a bit annoying with the template.

[yhmtsai]

same here and for the others.

[yhmtsai]

I think the file can go into unified one but you need to do dispatch on the header of factorization_helpers.hpp

[yhmtsai]

you may need to use GKO_KERNEL for the lambda function.
Maybe also for the actual kernel?

[MarcelKoch]

I feel like this complicates our unified setup again, and it's not using the unified kernels as intended.
We use the unified kernels, when the kernel implementation are identical, which is not the case here. I think it's close, but it would require some adjustments of the factorization kernels. If that's feasible, I would prefer to do it in another PR.

[yhmtsai]

Suggested change

	} // namespace gko
	} // namespace gko

nit

[yhmtsai]

sorry, I thought they are in the same namespace, but one is under kernel namespace and the other is under gko directly.
then the original one is better as it does not give any ambiguous function.
although it can only declare the two class you need, it's a bit annoying with the template.

[yhmtsai]

LGTM. I need to take my previous comment about the using namespace back because I thought they are in the same namespace, which is wrong.

[fritzgoebel]

LGTM!

[codecov]

Codecov Report

Attention: Patch coverage is 97.87234% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.09%. Comparing base (53c9fac) to head (c2aad74).
Report is 8 commits behind head on develop.

Files with missing lines	Patch %	Lines
omp/factorization/factorization_kernels.cpp	71.42%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1634      +/-   ##
===========================================
- Coverage    90.55%   90.09%   -0.47%     
===========================================
  Files          776      779       +3     
  Lines        63205    63336     +131     
===========================================
- Hits         57237    57061     -176     
- Misses        5968     6275     +307     

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[sonarcloud]

Quality Gate failed

Failed conditions
28.6% Duplication on New Code (required ≤ 20%)

See analysis details on SonarCloud