Add IL2M strategy

[Mathieu4141]

Hi!

I'd like to implement IL2M in avalanche, along with the metric that was proposed to show bias toward new classes.
reference paper: https://openaccess.thecvf.com/content_ICCV_2019/papers/Belouadah_IL2M_Class_Incremental_Learning_With_Dual_Memory_ICCV_2019_paper.pdf
code: https://github.com/EdenBelouadah/class-incremental-learning/tree/master/il2m

I don't know if there is already a plan to port this or if it's ok if I start working on that?

[vlomonaco]

@EdenBelouadah for sure wanted to port it, but I think she is focusing on #527 right now :-)
@EdenBelouadah, @AntonioCarta is it ok if @Mathieu4141 starts working on IL2M? @EdenBelouadah maybe you can review the PR!

[EdenBelouadah]

Hello,
Yesterday, I started working again on SIW (#527), and the plan was to pass after that to IL2M.
If you want, you can start working on IL2M, and I will join you later. Maybe by the end of this week of the beginning of the other.

[AntonioCarta]

Fine by me.

If you have any doubts feel free to ping me on the ContinualAI slack.

[Mathieu4141]

Great 😄

I started to write a metric for the Mean Scores comparison you made in the paper #683
The MeanScores metric can be reused for the plugin :)