Add two functions to allow dynamic weighting of samples per batch

There are no tests for this one and for this we should do it?

Is there any reference in the literature for this @amohammadi ?

Thanks

I learned this from @ageorge I don't know if he found it from somewhere.

There are tests but they are doctests as you can see above.

Well, the reference goes back to @onikisins, it seems to work well in practice for binary tasks.