Avoid double MultiIndex factorization in groupby index result

[mroeschke]

Description

From an offline discussion, it was discovered that a groupby with multiple keys experienced a slowdown somewhere between 24.02 and 24.12. An identified hot spot was the creation of the resulting MultiIndex where each resulting level undergoes factorization upon construction (which changed somewhere in between these releases to fix consistency bugs)

While the eager factorization is a new performance penalty, I identified that this factorization was being done twice unnecessarily when performing an agg. This PR ensures we avoid creating this MultiIndex until necessary and cache this MultiIndex result to avoid double factorization in the future

# PR
In [1]: import cudf
   ...: 
   ...: df_train = cudf.datasets.randomdata(nrows=50_000_000, dtypes={"label": int, "weekday": int, "cat_2": int, "brand": int})

In [2]:     target = "label"
   ...:     col = ['weekday', 'cat_2', 'brand']

In [3]: %%time
   ...: df_train[col + [target]].groupby(col).agg(['mean', 'count'])
   ...: 
   ...: 
# PR
CPU times: user 366 ms, sys: 109 ms, total: 474 ms
Wall time: 482 ms

#branch 25.02
CPU times: user 547 ms, sys: 112 ms, total: 659 ms
Wall time: 658 ms

Checklist

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