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This PR adds the possibility to specify a regret factor to increase the stepsize gamma at every iteration, if adaptive, before backtracking. Recent results provide convergence guarantees even without a maximum value on gamma [Theorem 14, arXiv:2208.00779v2].
This feature was implemented for ForwardBackward only. Although it does not seem a good fit for accelerated methods like ZeroFPR, PANOC, and PANOCplus, at least when based on quasi-Newton directions, it is unclear whether the FastForwardBackward solver could benefit from it. See discussion in #86 .
Practical performance seems to improve with values regret_gamma close to 1. Tests and references have also been updated accordingly.