[data] Simplify Operator.__repr__

[raulchen]

Why are these changes needed?

Currently, Operator.__repr__ prints the full DAG. It's too verbose.
Simplify it to only print the current operator. Added dag_string for the full DAG.

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[bveeramani]

I think we'll need to update this line to use dag_string:

ray/python/ray/data/_internal/execution/streaming_executor.py

Line 109 in 6287ebd

logger.info(f"Execution plan of Dataset: {dag}")

LGTM otherwise