This repo contains a Pytorch implementation of the SAC-X RL Algorithm [1]. It uses the Lunar Lander v2 environment from OpenAI gym. The SAC-X algorithm enables learning of complex behaviors from scratch in the presence of multiple sparse reward signals.

In addition to a main task reward, we define a series of auxiliary rewards. An important assumption is that each auxiliary reward can be evaluated at any state action pair. The rewards are defined as follows:

Auxiliary Tasks/Rewards

• Touch. Maximizing number of legs touching ground
• Hover Planar. Minimize the planar movement of the lander craft
• Hover Angular. Minimize the rotational movement of the lander craft
• Upright. Minimize the angle of the lander craft
• Goal Distance. Minimize distance between lander craft and pad

Main Task/Reward

• Did the lander land successfully (Sparse reward based on landing success)

Each of these tasks (intentions in the paper) has a specific model head within the neural nets used to estimate the actor and critic functions. When executing a trajectory during training, the task (and subsequently the model head within the actor) is switched between the different available options. This switching can either be done randomly (SAC-U) or it can be learned (SAC-Q).

The pictures below show the network architechtures for the actor and critic functions. Note the N possible heads for N possible tasks (intentions in the paper) [1].

Learning

Learning the actor (policy function) is done off-policy using a gradient based approach. Gradients are backpropagated through task-specific versions of the actor by using the task-specific versions of the critic (Q function). Importantly though, the trajectory (collection of state action pairs) need not have been collected using the same task-specific actor, allowing learning from data generated by all other actors. The actor policy gradient is computed using the reparameterization trick (code in model.py)

Learning the critic (Q function) is similarly done off-policy. We sample trajectories from a buffer collected with target actors (actor policies frozen at a particular learning iteration). The critic policy gradient is computed using the retrace method (code in model.py)

• Use the local/template.sh script to train lots of model variations. Or use train.py to train an agent directly.

• Python 3.6
• PyTorch 0.3.0.post4
• OpenAI Gym
• tensorboardX

[1] Learning by Playing – Solving Sparse Reward Tasks from Scratch.