Lately, I have been thinking deeply (pun intended) of Imitation Learning and its prospects for enabling fully autonomous robots and cobots. This will help us not do what we really do not want to do. Well, you might say, “look Sanjay, there is nothing groundbreaking about this idea and you are definitely not the first and the last to think about it”. And I would agree to it but what is interesting in my case is what transpired to lead me to this point of writing a blog post about it. It started during one of my conversations with my friends when I visited India a month and a half ago. We talked about if we can do something about the deeply disgusting inhuman job of Manual Scavenging with AI that unfortunately still exists all across the country. Certainly, with all the buzz around capabilities of deep reinforcement learning today (especially after it equipped AI with enough learnability to shatter the grand challenge of the game of GO), and predictions of the impending singularity through it, the idea of having some form of an intelligent robot that can clean our sewers and septic tanks should not too far-fetched, right. Anyway, it was not too hard to understand very soon that the premise of such exaggerated extrapolations to what AI in near future will do is definitely not on the horizon and definitely not so for a fully autonomous scavenging robot. But wait, given the maturity of supervised learning methods that we have attained today, maybe imitation learning can help here as all we apparently would need is memorize the mapping between the right action to take given the situations in the demonstrations. So here I am today, after reading about Learning from Demonstrations (LfD) for a few weeks now, writing about it which is incentivized by both disseminating what I understand about it at this point and validating that I actually do understand it (somewhat).
I plan to cover LfD in three separate blog-posts. In this first post, I’ll walk everyone through the what I understand by LfD, why it makes sense to me to invest time in it, and a technique called Behavioral Cloning. Later in the future, I’ll cover Inverse Reinforcement Learning and some important extensions of LfD.
Learning by imitation in human history has played a pivotal role in steering our evolution and continues to play a crucial role in shaping intelligent behavioural societies. Kids learn most of their daily life routines like cleaning, cooking by observing their caregivers. Hence the pursuit of a stronger AI seems to fall short without equipping the AI agents with the ability to learn useful behaviour by observing. Additionally, as more and more AI-powered systems will soon permeate into our societies in the form of self-driving cars, caregiving robots, cobots etc, this calling for devising ways for better learning by imitation makes more appeal as it opens up the possibility for even a layman person to tweak their robot’s behaviour by demonstrating their intention.
LfD (sometimes also called Imitation Learning) is an umbrella term used for any learning methodology that leverages demonstrations to learn any function of interest. The simple nature of many LfD techniques had prompted researchers for quite some time to leverage it for complicated tasks such as avoiding navigating through pedestrian trajectories (), helicopter control (), autonomous driving (), one-shot visual transfer () etc. In this section I will first develop perspective around why LfD is significant today, followed by formalizing LfD and different ways of approaching it, then talk about unique complications arising from the nature of LfD based techniques and finally talk briefly about getting around these unique LfD problems and extensions.
2. Why do we need LfD
Very broadly, LfD has three major advantages over other learning paradigms:
- Communication of intent is easy:: The major way of communicating intent is through reward functions but reward functions are hard to design. Given that our state of the art AI techniques can potentially do wireheading or reward hacking () and the ability of LfD techniques to open up tailoring of AI-powered systems, methods that leverage demonstrations seem more enticing.
- Finding a good control solution is hard:: Optimization (or credit assignment) is hard, and so is exploration. A lot of times the converged solution is not the best possible achievable answer. However, conveying the best solution through demonstrations is an easier task to do. This phenomenon even exists in the real world, as it had been observed with Fosbury Flop (, see figure below).
- Learning from scratch is time-consuming and dangerous:: Usually learning from scratch necessitates making a catastrophic error to know that it was bad. This is unacceptable, especially in the real world. However, demonstrations can be used to effectively communicate how to avoid such situations.
3. Formalizing LfD
Let and denote the states and actions respectively and be any trajectory or rollout. Let and be the trajectory distribution and the state distribution at time respectively induced by the policy . Finally, let be the overall state distribution induced by policy . The general goal of LfD is to find a parameterized learner policy such that some loss function is minimized with respect to the demonstrator policy on the state distribution induced by the learner, i.e.,
Usually, there is either limited or no access to and so a surrogate function that uses demonstrations is used instead. For the sake of simplicity here, I will keep using instead of the surrogate function. See figure below for an example.
4. Broad kinds of LfD
Very broadly LfD methods can be categorized into three kinds:
- behavioural cloning passively,
- active behavioural cloning with an interactive demonstrator,
- inverse reinforcement learning
In this subsection, I will cover the first two categories of LfD. In the subsequent blog posts, I will also talk about inverse reinforcement learning other important extensions of LfD such as third person imitation learning and domain transfer including the inherent challenges of LfD.
4.1 Behavioral Cloning Passively
It is the simplest of all LfD techniques where the learner tries to minimize the loss function in the demonstrator induced distribution of states or simply put learns the mapping between the state and action pair in the demonstrations fed to it.
This looks clean and intuitive. However, the applicability of behavioural cloning is terribly limited in the real world.
Why Behavioural Cloning is limited in its applicability::
Notice the difference between the induced state distributions in general LfD goal equation (first equation above) and Behavioural Cloning goal (second equation above). Usually there is a per time-step learner error in mimicking the demonstrator, which I will denote as , i.e. . As the subsequent states depend on the previous states and actions, this becomes a sequential decision-making problem that violates the i.i.d assumption of supervised learning. As a result, keeps accumulating which results in inconsistencies in the learner induced and the demonstrator induced distribution of states. The work in  and  showed that in a -step horizon of episodes, the accumulated error grows in the order of . See figure below for an example.
As a result, Behavioural Cloning techniques work well when the demonstrations exhaustively span across the whole state-space of interest, is not too bad and long-term planning is not necessary.
4.2 Active Behavioral Cloning with an Interactive Demonstrator
Interactive Demonstrator in the context of LfD is a demonstrator which can be asked for demonstrations at any state. Methods like SEARN (), DAGGER () and SMILe () showed that by repetitively doing Behavioural Cloning on the interactive demonstrator feedback on the learner induced distribution of states in an online fashion (see figure below) guarantees that the accumulation of errors does not exceed . The methods in this category are analyzed using reduction-based approaches ().
The algorithm below shows how the skeleton of interactive LfD techniques looks like in one of its most popular techniques called DAGGER ([12)] which works the best almost invariably and serves as an ideal benchmark for newer algorithms in its category. Note that are the number of iterations, the rate to decay demonstrator interjection in a rollout and number of time-steps in an episode respectively.
However, this category of LfD methods is not a silver bullet either. All interactive LfD methods assume the availability of an interactive demonstrator which is not always realistic. Moreover, they also have drawbacks that emanate from not having a notion of planning integrated into the control process such as treating all deviations equally (some of them could be catastrophic) and asking for demonstrations at every visited state even if they might not be required.
In conclusion, LfD is a promising technique without which our AI would be terribly impaired. Behavioural Cloning is a simple and intuitive technique to learn from demonstrations but comes with the catch that the learner can keep drifting away from the shown demonstrations. And whatever can go wrong does go wrong in the real world. In the future, I will write about relatively more robust methods for LfD like inverse reinforcement learning that attempts to learn the intent of the demonstrator behind the demonstrations. This has the potential to generalize better as the intent can be used to infer the right actions even at undemonstrated regions of the state-space.
I hope this post also helped you to think deeply (pun intended again) of LfD and whetted your appetite to know more.
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