Description

I was trying to implement t-SNE for mlpack and required the delta bar delta update rule given bellow.

I am currently using a callback to achieve the same result. It would be better if this was provided by ensmallen itself.

I can make a PR if there's interest for such a addition.

References

[1] R.A. Jacobs. Increased rates of convergence through learning rate adaptation. Neural
Networks, 1:295–307, 1988.