Low Rank - Linear, Quadratic problems API

[zoepiran]

Hi,
we noticed some inconsistencies in the API for solvers (Regular and Low Rank) between Linear and Quadratic problems and thought it may be useful to report them. Surely these may be a result of our misunderstanding and in that case we would be grateful for your clarifications.
Issues:
(1) epsilon: for a Linear problem one must provide it through the Geometry object whereas in Quadratic passed to the solver.
(2) Low Rank solver: two separate solvers in the Linear case, single solver for the Quadratic case.

We believe the code snippet below depicts the above in a straightforward manner.
Thank you in advance! :)

    if alpha == 0: # Linear problem
        ot_prob = ott.core.problems.LinearProblem(M)
        if epsilon: # regular (epsilon passed through Geometry M )
            solver = ott.core.sinkhorn.Sinkhorn()
        else: # Low rank (rank passed)
            solver = ott.core.sinkhorn_lr.LRSinkhorn(rank=rank)
    else: # Quadratic problem
        if alpha == 1: # GW
            ot_prob = ott.core.quad_problems.QuadraticProblem(geom_xx=C1,
                                                              geom_yy=C2,
                                                              fused_penalty=0)
        else: # FGW
            ot_prob = ott.core.quad_problems.QuadraticProblem(geom_xx=C1,
                                                              geom_yy=C2,
                                                              geom_xy=M,
                                                              fused_penalty=(1 - alpha) / alpha)
        [a one liner passing both epsilon and rank (whichever is not None implies usage)]
        solver = ott.core.gromov_wasserstein.GromovWasserstein(epsilon=epsilon, rank=rank)
    ot_gw = solver(ot_prob)```