Add PolarExpress Variant to Muon #1621
Description
This has already been suggested here #1602.
The PolarExpress (Amsel et al., 2025) is an optimal method to compute the polar decomposition of a matrix. The authors have demonstrated that their method shows consistent improvements over other methods when used with Muon, and addressed "finite-precision issues, making it practical to use in bfloat16".
In terms of implementation, it takes a similar form to Newton-Schultz but with iteration-dependent polynomial coefficients. Hence, it is a light-weight addition to Muon and fits nicely into the existing interface.
(I promise next time I will create the issue before submitting the PR)