add map-elites, with a brax ant example

[lerrytang]

Add a simple implementation of the MAP-Elites algorithm (ref1, ref2), with an example on the Brax Ant task. Users can implement their own behavior descriptor extractor, see an example in task/brax_task.py.

Example commands

python train_ant_map_elites.py --max-iter=3000
python train_ant_map_elites.py --max-iter=3000 --save-gif  # This can be slow

Example output

Note We plan to add more Quality Diversity methods, the implementations and related interfaces are subject to change.

[alantian]

LGTM! Before merging, please have a look at the one minor issue (see the comment).