Refactoring ideas regarding the preprocessor

[hyeok9855]

After reviewing the codes, I brought two possible refactoring ideas.

1. Move the parameter preprocessor in GFNModule to Sampler. This could separate env from GFNModule (since the preprocessor is defined in env) and pass the preprocessed states to the GFNModule. One drawback is that the input of GFNModule would not be State anymore.

2. There are two preprocessor files: src/gfn/preprocessors.py and src/gfn/gym/helpers/preprocessors.py. If this is not intended, it would be better to merge them into one file.

These are naive suggestions, so please feel free to comment, and let's discuss!

[josephdviviano]

Thanks for your ideas!

1. For 1 - I'm not sure where the states would be passed? I think we could also explore integrating the sampler completely within the GFNModule although I'm worried this might be a footgun down the line if some method requires that modularity. @saleml thoughts?
2. I don't love src/gfn/gym/helpers in general. I feel like the "helpers" should either be integrated into the main library or the individual gym files, and the whole folder should be removed.

[hyeok9855]

I think we could also explore integrating the sampler completely within the GFNModule

Yes! Let's discuss this in #195 about it :)