Carry over optimiser state in update() methods

[ablaom]

This PR corrects an issue whereby optimiser "state" (eg, the momentum) is not carried over in a warm-restart. For example, the following test (added in this PR) now passes:

            using MLJBase, MLJFlux
            X, y = make_regression()
            model = NeuralNetworkRegressor() 
            
             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

[ablaom]

The tests referred to in the opening post pass on the CPU. However, they do not pass on the GPU for reasons I don't understand at this time.

Since the current PR does fix a bug, I am going to merge this and postpone the other half of the bug to a new issue.

I've marked the failing tests (which do not previously exist) as broken.