Fix set of hyperparameters when optimizing

[mo-fu]

When optimizing hyperparameters I may want to fix a specific parameter.
I.e., do I want to train bonsai or parabel variants of omikuji.

This could possibly configurin by not optimizing hyperparameters defined int the project.
I had a look at Optuna a while back they don't allow this by default. The option I see is to wrap the trial in a proxy to handle this.

[osma]

This makes sense, and I've had the same thought for example about fastText (where hyperopt isn't supported yet) - sometimes it would make sense to fix e.g. loss=hs when performing hyperparameter optimization, because the other options would be way too slow.

However, I'm not sure if "not optimizing hyperparameters defined in the project" is the best way - how about instead being able to specify the fixed parameters on the command line? This could be done using the --backend-param option, which is already implemented for e.g. the suggest command.