Add eps and amsgrad options to Adam Optimizer

[zeyugao]

Expose more options in AdamOptions

Fix #145

However it will break the adamw and adam interface under src/nn/optimizer.rs. Should I create a new functions like adam_new?

[LaurentMazare]

Thanks for the PR, I've made further changes to preserve the current api and the newly added field can still be configured via a builder pattern. I think I'll lilkely remove the nn::adam and nn::adamw functions altogether as the intended way to setup the optimizer would be as follows, though I want to think a bit more about it before removing these.

let opt = Adam::default().eps(1e-6).amsgrad(true).build_copt(...)