Make stochastic solvers fit in this benchmark

[tomMoral]

Now that we use SufficientProcessCriterion to stop the benchmark and that we can have multiple loss at once, there is no real gain to have separate benchmarks for logreg_l2 with batch and stochastic solvers. We should thus merge benchopt/benchmark_stochastic_logreg_l2 and this benchmark. To do this, we should:

• port SGD solver in this benchmark
• add a notion of train/test losses in objective.py
• improve the plotting utils for multi value loss functions to get train/test graphs.
• close the benchopt/benchmark_stochastic_logreg_l2 repo

Moreover, to make this benchmark more inline with literature practices, we should try and reproduce the following figures:

• From Schmidtt et al. 2014
  

• From Leblond et al. 2017