Robustness fix to make sure we ignore tags with known names, but
which are nested inside of unknown tags. For example we don't want
to parse the third period in:
<Period>...</Period>
<Period>...</Period>
<CustomTag>
<Period>...</Period>
</CustomTag>
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=217101630
