[MLA-1474] detect recursion on Agent methods and throw

[chriselion]

Proposed change(s)

It was possible for user implementations of Agent.CollectObservations() and Agent.OnEpisodeBegin() to get stuck in infinite recursion if EndEpisode was called from them. We now detect the recursive calls and throw an exception.

This generalizes the approach from a similar issue on Academy.

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

Fixes some (but not all) of the concerns in #4558
https://jira.unity3d.com/browse/MLA-1474
#4227

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

Other comments

[andrewcoh]

LGTM

[vincentpierre]

Looks, good. I added one suggestion.

[vincentpierre]

I think it would be more general if instead of passing methodName as argument, we passed the whole error instead. The constructor would look like :

new RecursionChecker("CollectObservations() was called recursively. Make sure there are ...");

This would make the error messages more clear, and the code future proofer, but I do not have a strong opinion on this.

[chriselion]

I don't really know how to make the errors messages more clear and still catch all possible permutations that users could set up. I'm afraid if we say something like "don't call EndEpisode from CollectObservations" and someone manages to hit CollectObservations recursively some other way, it will be more confusing.