Add support for early stopping

[wayhoww]

I am using the SVD++ algorithm. I found that the process of training is hardly coded and the weight will be re-initialized before training once we call the fit for the second time. So I cannot come up with any ideas to do early stopping with official surprise. Could we add support for early stopping, or is there some reason for not adding it ?

[NicolasHug]

Hi @yslking , sorry for the late reply. I don't have as much time as I used to, so I'm mostly just maintaining the project now; I'm unable to commit to new features, sorry