Remove optuna default values

[nils-werner]

A big chunk of the parameters in /configs/hparams_search/mnist_optuna.yaml were the default values from the documentation. I don't think these values need to be repeated here, right?

[ashleve]

I like having the default parameters there, since the template purpose is also educational. Having explicit statement of all the possible parameters helps to realize the possibilities. For example, it's important to be aware about the n_startup_trials param since it determines how many random runs will be executed at the beginning of search, before the optimization sampling starts.

[ashleve]

Although it should probably be better documented with some comments

[ashleve]

I added some explanatory comments and restored storage, study_name, n_jobs, n_startup_trials, since I believe those are quite important. Thanks for the PR!