[BUG] Multi index groupby with sort=True inconsistent with pandas

[ChengjieLi28]

Describe the bug

Multi index groupby with sort=True inconsistent with pandas.

To Reproduce

To help us reproducing this bug, please provide information below:

1. Your Python version: 3.9.12
2. The version of Mars you use: latest
3. Versions of crucial packages, such as numpy, scipy and pandas: follow mars
4. Full stack of the error.
   No error.
5. Minimized code to reproduce the error.

In [8]: mdf.groupby(["b", "c"], sort=True)["a"].agg(['nunique', 'mean']).execute()
100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 100.0/100 [00:00<00:00, 1064.07it/s]
Out[8]:
     nunique      mean
b c
a d        7  3.636364
b b        7  4.000000
c d        6  5.000000
d b        3  5.000000
a a        5  5.166667
  c        2  4.000000
c a        6  4.285714
  c        4  5.166667
a b        5  2.857143
b d        2  3.500000
c b        5  5.300000
d a        1  7.000000
  c        5  4.125000
b a        5  2.666667
  c        3  6.666667
d d        8  4.230769

In [9]: df.groupby(["b", "c"], sort=True)["a"].agg(['nunique', 'mean'])
Out[9]:
     nunique      mean
b c
a a        5  5.166667
  b        5  2.857143
  c        2  4.000000
  d        7  3.636364
b a        5  2.666667
  b        7  4.000000
  c        3  6.666667
  d        2  3.500000
c a        6  4.285714
  b        5  5.300000
  c        4  5.166667
  d        6  5.000000
d a        1  7.000000
  b        3  5.000000
  c        5  4.125000
  d        8  4.230769

Expected behavior

A clear and concise description of what you expected to happen.

Additional context

mdf.groupby(["b", "c"], sort=True)["a"].agg(['nunique', 'mean']).execute().fetch().sort_index()
is same as pandas result.