BUG: Partial dtypes doesn't work when mixing MultiIndex and non-MultiIndex columns

[dchigarev]

Modin version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest released version of Modin.

• I have confirmed this bug exists on the main branch of Modin. (In order to do this you can follow this guide.)

Reproducible Example

import modin.pandas as pd

df = pd.DataFrame({"a": [1, 1, 2, 2], "b": [3, 3, 4, 4], "c": [4, 3, 2, 1]})
res = df.groupby("a").agg({"b": ["min", "std"], "c": ["min", "std"]})
res = res.reset_index()
print(res.dtypes) # AssertionError

Issue Description

After reset_index() partial dtypes will look like this, which will cause the failure at materialization stage:

ModinDtypes:
        value type: <class 'modin.core.dataframe.pandas.metadata.dtypes.DtypesDescriptor'>;
        value:
        DtypesDescriptor:
        known dtypes: {'a': dtype('int64')}; <--- non MultiIndex label
        remaining dtype: None;
        cols with unknown dtypes: [('b', 'min'), ('b', 'std'), ('c', 'min'), ('c', 'std')]; <--- MultiIndex labels
        schema is known: False;
        has parent df: True;
        columns order: {0: 'a', 1: ('b', 'min'), 2: ('b', 'std'), 3: ('c', 'min'), 4: ('c', 'std')};
        know all names: True

Expected Behavior

to work

Error Logs

Traceback (most recent call last):
  File "modin/t4.py", line 6, in <module>
    print(res.dtypes)
  File "modin/modin/pandas/base.py", line 4167, in __getattribute__
    attr = super().__getattribute__(item)
  File "modin/modin/pandas/dataframe.py", line 325, in dtypes
    return self._query_compiler.dtypes
  File "modin/modin/core/storage_formats/pandas/query_compiler.py", line 321, in dtypes
    return self._modin_frame.dtypes
  File "modin/modin/core/dataframe/pandas/dataframe/dataframe.py", line 387, in dtypes
    dtypes = self._dtypes.get()
  File "modin/modin/core/dataframe/pandas/metadata/dtypes.py", line 832, in get
    self._value = self._value.to_series()
  File "modin/modin/core/dataframe/pandas/metadata/dtypes.py", line 436, in to_series
    self.materialize()
  File "modin/modin/core/dataframe/pandas/metadata/dtypes.py", line 418, in materialize
    assert len(self.columns_order) == len(self._known_dtypes)
AssertionError

Installed Versions

Replace this line with the output of pd.show_versions()