Package extensions for Flux

[HenriDeh]

Hello @jeremiahpslewis @findmyway and any other who wishes.

The upcoming 1.9 version will introduce the package extensions feature, which RL.jl could greatly make use of. I figured it would be a good idea to discuss here the packages that we want to extend.

Here are my first ideas:
[] CUDA : the most obvious one, it's heavy and lengthy to install. People who don't need it would be glad to avoid that I believe.
[] Flux : not needed for tabular RL, therefore wasteful to load for that case.

I wanted to add SimpleChains but as it is not currently used by the package it is for another time. One day I'd like it if one could use these fast NNs.

Let me know your other suggestions.

[findmyway]

I've experimented with this feature a lot recently. We desperately need it to keep RL.jl lightweight and easier to maintain.

I estimate a lot of work is required to separate CUDA.jl and Flux.jl, but this is the right direction to go. 🏃

[jeremiahpslewis]

@HenriDeh I don't know anything about this feature, but it sounds like a great idea!

[jeremiahpslewis]

Note: CUDA is no longer a dependency, Flux is, could be separated out into an extension