Avoid calling logging.debug on every element

This call can be quite expensive, even if logging is disabled.
Instead, gate this on a boolean set at start().

This is slightly less flexible as debug logging cannot be
dynamically toggled for these log messages between bundles.

----Release Notes----
Performance improvements.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117854898

Author: robertwb