Add Visual3DBall scene

[dongruoping]

Proposed change(s)

Add a visual version of 3DBall. The agent takes 84x84 camera frame with frame stacking of 4 as input.
A Use Vec Obs option is added to 3DBall agent.

This can be served as a good example to demonstrate the use of visual frame-stacking for the newly added stacking feature for visual/compressed observation

This PR will wait until #4476 is merged.

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

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

[chriselion]

Not sure if it matters, but if you look at the Camera preview, you can see the neighboring agents:

Can you check in a model file (maybe once you finish with the other PR and enable stacking)?

[dongruoping]

Oops I've fixed it when running experiment but somehow didn't include it. It should be fixed now.
And sure I'll add a model file.

[vincentpierre]

I think he colors of the agents should not be green. Green is reward in our informal color coding, blue and purple are agent color.
Also, make sure you give a model of the agent.

[dongruoping]

I think he colors of the agents should not be green. Green is reward in our informal color coding, blue and purple are agent color.
Also, make sure you give a model of the agent.

Should I use either blue or purple? I did that just to make a difference between the original 3DBall and the Visual one.

[vincentpierre]

Ok I'll run more times to get a better number. This worked when I tried last time.

Does the scene use stacking by default or do I need to change something?

[dongruoping]

Ok I'll run more times to get a better number. This worked when I tried last time.

Does the scene use stacking by default or do I need to change something?

Oh so this one is still waiting for the stacking PR to merge (thus I made this one as draft) and will add default stacking after that.
Sorry I didn't made it clear.

[vincentpierre]

I would add a Tooltip to explain that this needs to be checked in the 3DBall scene but unchecked in the Visual3DBall scene (since it is a public inspector).