[BugFix] Return a list of ntypes/etypes

[chwan1016]

Description

The comment of function load_partition shows that two return variables ntype and etype are lists. After running the code, I notice that the actual types are dictionaries. I guess returning lists is preferred, otherwise the comment should be fixed.

Note that there is no test that is related to this change.

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented
• To the my best knowledge, examples are either not affected by this change,
  or have been fixed to be compatible with this change
• Related issue is referred in this PR

Changes

[zheng-da]

It seems there is some inconsistency between the comments and the code. Thanks for fixing the problem.
By checking how load_partition and load_partition_book are used, I think it's better to make the returned results by these two functions consistent. Please check out how these two functions are used here.
I think our option is either returning dicts in both functions and changing the docstring of load_partition, or returning lists and changing the docstring of load_partition_book.