Hot Load and Chunked Save of Demo Files

[ervteng]

Proposed change(s)

Adds two new IL improvements: allow for adding additional .demo files during a training run, and saves new demos from the DemonstrationWriter in chunks of Num Steps to Record size.

You can combine these two features to do online IL by pointing the writer and the GAIL/BC YAML to the same folder, but no explicit way of doing online has been added.

Also fixed a longstanding bug that if multiple demo files are in a folder, they are all treated as one long appended file, which is not correct as the final obs/action pair will be joined with the first one from the next file.

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

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