Add support for `OptimizerParamGroup`

OptimizerParamGroup is used to set different learning rates, weight decay rates, and other optimizer properties differently for different model parameters.

Exmaple: AdamW(vector<OptimizerParamGroup> param_groups, AdamWOptions defaults)

[NiklasGustafsson]

@dsyme, @lostmsu – we can have it both ways:

var pgs = new ASGD.ParamsGroup[]
{
    new (lin1.parameters(), new () { LearningRate = 0.005f }),
    new (lin2.parameters())
};

var optimizer = torch.optim.ASGD(new ASGD.ParamsGroup[]
{
    new () { Parameters = lin1.parameters(), Options = { LearningRate = 0.005f } },
    new () { Parameters = lin2.parameters() }
});

let pgs = [|
     SGD.ParamsGroup(Parameters = model.parameters(), Options = SGD.Options(momentum = 1.0, dampening = 0.5));
     SGD.ParamsGroup(model.parameters(), momentum = 1.5, dampening = 0.1)
|]

let optimizer = SGD([|
    SGD.ParamsGroup(model.parameters(), momentum = 1.0, dampening = 0.5);
    SGD.ParamsGroup(model.parameters(), momentum = 1.5, dampening = 0.1)
|], lr)

[NiklasGustafsson]

Which can be further simplified by renaming the optimizer class. This should be alright, since the name of the factory method that should be the natural way of creating an optimizer doesn’t change:

            var pgs = new ParamsGroup<ASGD.Options>[]
            {
                new () { Parameters = lin1.parameters(), Options = { LearningRate = 0.005f } },
                new () { Parameters = lin2.parameters() }
            };

I’ll need to check how this looks in F#, too. Just as important to get the F# UX right…