Fine-tuned w1 and w2

[Expertium]

See #137 (comment)

Of course, I want you to implement this in the optimizer.

[L-M-Sherlock]

LGTM, I will re-benchmark FSRS-5-pretrain tomorrow.

[Expertium]

I don't think it will affect the metrics. Interpolation is needed only if a button or two have never been used during the first review. But if those buttons have never been used, that means their respective S0 will also never be used!
In other words, we cannot see any difference using our current benchmarking setup. But there will be a difference for those real users who have never used a button and are now starting to use it.