feat: allow for loading image datums without rgb

[chrisochoatri]

This adds a flag to base_dataset and synchronized scene to optionally skip loading the raw image data in a camera datum. The rgb key will contain a None value. The expected use case is when loading autolabels and the raw image data is not needed.

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This change is 

[wadimkehl]

@chrisochoatri Any movement on this?