text: Title: BUG: multi-indexed dataframes' shape
  Body: ### Describe the bug
  The shape of a multi-indexed dataframe is incorrect.
  
  ### To Reproduce
  In Xorbits,
  ```python
  df = pd.DataFrame({"foo": (1, 2, 3), "bar": (4, 5, 6)})
  c = df.rolling(window=2).agg('corr')
  c.shape
  ```
  you got `(3, 2)`.
  
  But in pandas, you got `(6, 2)`.
  
  