Inference agent fix, improvements when using NCCL backend, and other minor improvements.

[Lucaweihs]

This PR makes the following changes:

• Fixes some problems with the inference agent that occurred when resetting the agent.
• Adds a read timeout to calls to vector sampled tasks so that the training process will exit correctly if the vector sampled tasks fails to respond for some amount of time (currently 1 minute but this may be too low).
• Adds experimental code to restart vector sampled tasks if an environment ever happens to throw an uncaught error. This functionality is disabled by default.
• Adds a new habitat experiment and HM3D support.
• Improves the vision sensor to ensure that it copies images before modifing them (necessary for the new version of ai2thor)
• A few other minor fixes.

[lgtm-com]

This pull request introduces 1 alert when merging 21b663a into 474fb84 - view on LGTM.com

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[lgtm-com]

This pull request introduces 1 alert when merging 481c449 into 474fb84 - view on LGTM.com

new alerts:

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[lgtm-com]

This pull request introduces 1 alert when merging 1d3c64a into 474fb84 - view on LGTM.com

new alerts:

• 1 for Unused import