Keyword-only link argument in GradientMethod

[niboshi]

Improves #3488 by making link argument keyword-only.

optimizer = chainer.optimizers.SGD(model)  # does not work

In #3488, this code does not work and dangerous because model is treated as a hyperparameter.
In this PR, user must explicitly specify link= keyword.

optimizer = chainer.optimizers.SGD(link=model)

Also, type check on hyperparameter assignment is implemented, so the former code raises TypeError.

I wondered which is better keyword, link or model, but as Optimizer.setup() method has link argument, I chose link.

[toslunar]

I suppose this change makes Hyperparameter less flexible.

[niboshi]

Do you mean you may want to set a hyperparameter which is not an array nor a scalar?

[toslunar]

Yes.
I've encountered the case I'd like to pass a string argument to an optimizer, which syncs parameters 'soft' or 'hard'. To be honest, my final code didn't pass the string, and I've not set a string value to a hyperparameter (because the optimizer was not an instance of GradientMethod.)

[niboshi]

So, we need to decide to which extent Hyperparameter accepts as a value.

The most conservative option is to accept anything but Link, but in this case we would need to add such rules in the future depending on how it is used. I don't think that's a good choice.

I think the minimum is scalars and ndarrays (as in the current code).
Maybe str is reasonable, too.

[niboshi]

Now that this argument has been removed (See #4141), I'm closing this PR.