Add --no-replace to datasets_download.py

[aclegg3]

Motivation and Context

Add --no-replace to datasets_download.py to skip existing data automatically. Should fix use-cases with automated download that are run multiple times, currently requiring re-download with --replace or crashing due to user prompt.

How Has This Been Tested

Locally.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.