[MRG] Allows to build NNs in A2C from a string equal to the function's name

[riccardodv]

Allows to build value_net_fn and policy_net_fn from a string using rlberry.utils.factory.load. This is useful when you want to
call model_factory_from_env to build the architectures from a .yaml file according to args specified therein

[riccardodv]

I think I got some warning but I am not sure if I should worry. It says stuff like "Added line #L117 was not covered by tests". Do you know why?

[TimotheeMathieu]

It is not a big problem it is just that in the test for A2C, policy_net_fn was always the default I guess so it does not test policy_net_fn which is string or callable.
It would be nice to include a small test on this, maybe doing one iteration on some environment to check that there is no Runtime error. You could do this with @pytest.mark.parametrize for instance, it is clean (example of use in

rlberry/rlberry/tests/test_agent.py

Line 53 in 85176de

@pytest.mark.parametrize("agent", FINITE_MDP_AGENTS)

).

[riccardodv]

@TimotheeMathieu I added some checks and is fine now. Thank you!