There are 5 repositories under nonlinear-least-squares topic.
A large scale non-linear optimization library
A library for differentiable nonlinear optimization
IMU + X(GNSS, 6DoF Odom) Loosely-Coupled Fusion Localization based on ESKF, IEKF, UKF(UKF/SPKF, JUKF, SVD-UKF) and MAP
Advanced Technique for Curve Fitting
An optimizer of robustified Non-Linear Least Squares problems
Fortran bindings to libdogleg - a large-scale nonlinear least-squares optimization library
Solve nonlinear systems of equations and nonlinear least squares in Julia
Least squares adjustment software