GPU-friendly truncation implementations

[lkdvos]

This is an attempt to get rid of the scalar-indexing oriented approach, and instead do more global operations.
Definitely still WIP, and on CPU there are definitely various optimizations that can be applied if needed.
I do wonder about the performance a bit, as I would actually expect that for a large number of sectors this might just be faster.

Some possible optimizations:

• for UniqueFusion, finding the nth value is simply partialsortperm(values, n; by, rev), avoiding the need to allocate the full permutation vector
• for CPU, cumsum + findlast can be replaced by a loop to avoid some intermediate allocations

[codecov]

Codecov Report

❌ Patch coverage is 91.56627% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/tensors/sectorvector.jl	50.00%	4 Missing ⚠️
src/factorizations/adjoint.jl	75.00%	3 Missing ⚠️

Files with missing lines	Coverage Δ
ext/TensorKitCUDAExt/TensorKitCUDAExt.jl	100.00% <ø> (ø)
ext/TensorKitCUDAExt/truncation.jl	100.00% <100.00%> (ø)
src/factorizations/diagonal.jl	72.72% <100.00%> (+6.06%)	⬆️
src/factorizations/truncation.jl	94.83% <100.00%> (+6.90%)	⬆️
src/factorizations/adjoint.jl	73.07% <75.00%> (-1.35%)	⬇️
src/tensors/sectorvector.jl	40.57% <50.00%> (-7.81%)	⬇️

... and 1 file with indirect coverage changes

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[Jutho]

I think that implementation is very clean. Not sure I can explain all the errors. Some seem to originate from the eigenvectors of a DiagonalTensorMap also being diagonal, which is being changed, right? But other errors I cannot directly explain without running the code locally.

[lkdvos]

I will have a look later, I have a local branch to fix the diagonal implementations already, we'll see what remains after.
I'll also try if it actually works on GPU, and get a similar implementation going for the truncerror version.

[kshyatt]

I think this one still needs a little work to get it over the finish line, but then we should be able to really try CUDA with the TNR and PEPS stuff

[github-actions]

Your PR no longer requires formatting changes. Thank you for your contribution!

[lkdvos]

wait please don't I'm working on this 😆

[kshyatt]

I'll stop I just wanted to fix formatting and rebase!!!

[lkdvos]

Hahah but the rebase screws up the git history, now I gotta see how much git magic I know to fix my local branch

[kshyatt]

Oh no I'm sorry!