add option for Burst inference

[chriselion]

Proposed change(s)

Followup from slack with Florent: Add InferenceDevice.Burst and make it the default. I don't think changing the existing behavior of InferenceDevice.CPU in a minor version is allowed though.

Also need to keep CPU as the default for Agent.SetModel:

ml-agents/com.unity.ml-agents/Runtime/Agent.cs

Lines 586 to 589 in 729b4cf

	public void SetModel(
	string behaviorName,
	NNModel model,
	InferenceDevice inferenceDevice = InferenceDevice.CPU)

for now.

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

https://jira.unity3d.com/browse/MLA-1763
https://jira.unity3d.com/browse/MLA-1765 (followup to break the behavior in the next major version)

Types of change(s)

• New feature

Checklist

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

[chriselion]

Prefab changes coming soon.