WIP: Add MPO in zoo

[HenriDeh]

I'm opening this as a draft so discussions are possible early.
This implements the MPO algorithm from this paper and its improved version
PR Checklist

• Add docstrings
• Handle the case of a discrete actor. For this, I was wondering if a DiscreteNetwork akin to GaussianNetwork may be a better approach than considering that if a GN is not used, then it must be a Discrete actor.
• Add some tests
• Does this handle distributed environments?
• Handle legal action masks
• Decide default HPs
• Make experiments with each network
• Remove normalizer from networks ?
• Make a dedicated doc page
• Fix GPU (changes in RLTrajectories needed)

[HenriDeh]

There we go, it's finally done. This PR adds MPO, you can find details in the dedicated doc page. It supports Categorical, Gaussian, and Full Covariance Gaussian policies. Compared to the MPO algorithm described in the related paper (see above), it does not support two main things:

• It uses 1-step TD learning to update the critic network, whereas the paper uses retrace. Implementing retrace is a WIP.
• It does not support distributed learners with gradient pooling. This is for later.

I implemented three experiments in the test suite, one for each type of policy. They all learn a perfect CartPole policy in less than a minute using only a CPU, at least on my computer.