[FutureWarn] Fix FutureWarning in CategoricalTensorMapper.

[drivanov]

The following FutureWarning is fixed:

test/data/test_mapper.py::test_categorical_tensor_mapper
  /usr/local/lib/python3.10/dist-packages/torch_frame/data/mapper.py:113: FutureWarning: Series.__getitem__ 
treating keys as positions is deprecated. In a future version, integer keys will always be treated as labels 
(consistent with DataFrame behavior). To access a value by position, use `ser.iloc[pos]`
    ser = pd.Series(self.categories[index].index)