$7.$ Implement a rating prediction model based on the similarity function r$($u$,$ i$)=$ R$$ i $+$ P j in Iu$\backslash \{$i$\}($Ru$,$j $$ R$$ j $)$ Sim$($i$,$ j$)$ P j in Iu$\backslash \{$i$\}$ Sim$($i$,$ j$),($there is already a prediction function similar to this in the provided example code, you can either start from scratch or modify an existing solution$).$ Report the MSE $($on the test set$)$ of this rating prediction function when Sim$($i$,$ j$)=$ Jaccard$($i$,$ j$)\mathrm{.}1$