Actuator Module self contained without other changes.

[surfnerd]

Proposed change(s)

Add the Actuator abstraction to ML-Agents as a self-contained module in order to avoid having an enormous pull request.

Notable Additions:

• IActionReceiver
  • Low-level interface that defines how RL actions can be passed to an object and how those actions (if discrete) can be masked.
• IActuator (inherits from IActionReceiver)
  • The Actuator interface that describes the data and function of the actuator.
  • Contains the ActionSpaceDef for both continuous and discrete actions
• IDiscreteActionMask (Passed to IActionReceiver)
  • The interface that allows different IDiscreteActionMasks to be implemented
• ActionSegment
  • A struct that allows for accessing an array of actions at offsets rather than copying sub-arrays around.
• ActuatorManager (an interface could probably be extracted from this class)
  • Handles the majority of the actuator logic for an Agent including:
    • buffer management
    • event life-cycle
      • writing to the action mask
      • executing actions
      • general bookkeeping of other metadata about the Actuators.

Questionable names:

• ActionSpaceDef
  • Defines a specific action space for an actuator (i.e. discrete and continuous at the moment.)
• ActuatorDiscreteActionMask
  • Keeps an action mask for all of the actuators' actions.

TODO:

• Docs

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

https://jira.unity3d.com/browse/MLA-789

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

[vincentpierre]

I did not see the BufferedDiscreteActionMask mentioned in the PR comment, is it still in the PR?
I made some initial comments but I need to review it some more.

[chriselion]

A few final nits, but looks good!