DCG-MAP-Elites

[maxencefaldor]

Add DCG-MAP-Elites algorithm to QDax.

This PR introduces:

• a new method for MAP-Elites repertoire, that enables to samples individuals with their corresponding descriptors.
• a new output extra_info for Emitter.emit methods that is similar to the extra_scores of the scoring function, and that enables to pass information from the emit step to the state_update (necessary for DCG-MAP-Elites).
• a new DCGTransition that add desc and desc_prime to the QDTransition.
• descriptor-conditioned TD3 loss, descriptor-conditioned scoring functions, descriptor-conditioned MLP
• two new reward wrappers to clip and offset the reward (necessary for DCG-MAP-Elites).