fix multicategorical stype inference and add test case

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 76.47059% with 4 lines in your changes missing coverage. Please review.

Project coverage is 93.60%. Comparing base (ee98b87) to head (eb535f2). Report is 51 commits behind head on master.

Files with missing lines	Patch %	Lines
torch_frame/utils/infer_stype.py	66.66%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #420 +/- ## ========================================== + Coverage 93.52% 93.60% +0.08% ========================================== Files 124 126 +2 Lines 6456 6556 +100 ========================================== + Hits 6038 6137 +99 - Misses 418 419 +1 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.