Implement TensorKit's tsvd! reverse-rule for :fixed differentiation mode

[pbrehmer]

Here we adapt tsvd! and FixedSVD such that the :fixed differentiation mode can also use TensorKit's SVD adjoint. For context, the TensorKit.tsvd! reverse-rule requires the full, untruncated SVD which is currently not passed along the CTMRG algorithm. To change this, tsvd! needs to return the untruncated decomposition as well. Hopefully, TensorKit's reverse-rule will eliminate some of the instabilities of KrylovKit's SVD reverse-rule.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 54 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/utility/svd.jl	0.00%	46 Missing ⚠️
src/algorithms/ctmrg/projectors.jl	0.00%	4 Missing ⚠️
src/algorithms/ctmrg/simultaneous.jl	0.00%	4 Missing ⚠️

Files with missing lines	Coverage Δ
...rithms/optimization/fixed_point_differentiation.jl	3.19% <ø> (-91.71%)	⬇️
src/algorithms/ctmrg/projectors.jl	8.57% <0.00%> (-86.89%)	⬇️
src/algorithms/ctmrg/simultaneous.jl	1.72% <0.00%> (-96.46%)	⬇️
src/utility/svd.jl	1.86% <0.00%> (-84.05%)	⬇️

... and 32 files with indirect coverage changes

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