Fix conjugation in Newton-Schulz iterator and update tests for comple…

[eirikfagerbakke]

Adds support for complex parameters in Muon, by adding .conj() to Newton-Schulz and the adaptive scaling step.

Also adds two tests. The first checks that the Newton-Schulz algorithm is able to make the updates orthogonal for real parameters and unitary for complex parameters. The second tests the Muon optimizer on a simple complex optimization problem.

Fixes #1494.

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