BUG: Groupby object does not implement len() method

[qianduoduo0904]

Describe the bug

Groupby object does not implement len() method.

To Reproduce

To help us to reproduce this bug, please provide information below:

1. Your Python version
2. The version of Xorbits you use
3. Versions of crucial packages, such as numpy, scipy and pandas
4. Full stack of the error.
5. Minimized code to reproduce the error.

In [1]: import xorbits.pandas as pd

In [2]: df = pd.DataFrame({'a': ['a', 'b', 'a', 'c'], 'b': [0.1, 0.2, 0.3, 0.4], 'c': ['aa', 'bb', 'cc', 'aa']})

In [3]: grouped = df.groupby('b')

In [4]: len(grouped)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[4], line 1
----> 1 len(grouped)

File ~/Workspace/xorbits/python/xorbits/core/data.py:267, in DataRef.__len__(self)
    263             raise TypeError(
    264                 f"object with shape {self.data._mars_entity.shape} has no len()"
    265             )
    266 else:
--> 267     raise TypeError(f"object of type '{self.data.data_type}' has no len()")

TypeError: object of type 'DataType.dataframe_groupby' has no len()

Expected behavior

__len__() implementd.

[RayJi01]

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