GridSensor Observation Stacks Greater Than 1 Throws Exception

[cm107]

Describe the bug
Using an observation stack of 1 works for GridSensor, but using a stack greater than 1 results in a UnityObservationException.

To Reproduce
Steps to reproduce the behavior:

1. Go to the FoodCollector scene in the examples folder.
2. Select all "Agent" game objects in the hierarchy.
3. Change the Observation Stacks value from 1 to 4.
4. Try training.

Console logs / stack traces

mlagents_envs.exception.UnityObservationException: Compressed observation and its mapping had different number of channels - observation had 15 channels but its mapping had 24 channels

Screenshots

Environment (please complete the following information):

• Unity Version: Unity 2020.3.4f1
• OS + version: Ubuntu 18.04.6 LTS
• ML-Agents version: release_19
• Torch version: 1.8.1
• Environment: 0.28.0 (from release_19 branch)