Avoid factorization in MultiIndex.to_pandas

[mroeschke]

Description

This also uncovered a bug in DataFrame.rename where the underlying MultiIndex ColumnAccessor was not being replaced

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

I'm not sure if I follow what this comment means.

Is the np.iinfo.min coming from libcudf? (More generally, where is that value coming from?)

Should this be a "replace value" rather than a "clip" call?

Should we prefer to call something from libcudf instead of cupy?

[mroeschke]

I think it's coming from libcudf indirectly because it's being used in MultiIndex.__init__:

cudf/python/cudf/cudf/core/multiindex.py

Line 192 in e03623a

code[code == -1] = np.iinfo(size_type_dtype).min

Should this be a "replace value" rather than a "clip" call? Should we prefer to call something from libcudf instead of cupy?

Good point. A "replace value" would be more suitable here. I was kinda blindly following how MultiIndex.codes generates cupy arrays and working off that

[mroeschke]

Mind re-reviewing when you have a chance @bdice?

[mroeschke]

/merge