Resolve issue around broken tests for optimiser state on GPU

[ablaom]

context: #86.

The following, which passes on the CPU, fails on the GPU for reasons I do not understand. It corresponds to the tests marked as "broken" at time of posting on buildkite CI.

           accel=CUDALibs()   # passing for CPU1()

            using MLJBase, MLJFlux
            X, y = make_regression()
            model = NeuralNetworkRegressor(accleration=accel) 
            
             model.epochs = 1
             mach = machine(model, X, y);

             # two epochs in stages:
             Random.seed!(123)
             fit!(mach, verbosity=0);
             model.epochs = model.epochs + 1
             fit!(mach, verbosity=0);
             l1 = MLJBase.report(mach).training_losses[end]

             # two epochs in one go:
             Random.seed!(123)
             fit!(mach, verbosity=1, force=true)
             l2 = MLJBase.report(mach).training_losses[end]

             @test l1 ≈ l2