[BACKPORT] Implements DataFrameGroupBy.sample (#1994) (#1995)

Author: wjsi

docs/source/reference/dataframe/groupby.rst

@@ -71,6 +71,7 @@ application to columns of a specific data type.
    DataFrameGroupBy.cummin
    DataFrameGroupBy.cumprod
    DataFrameGroupBy.cumsum
+   DataFrameGroupBy.sample
 
 The following methods are available only for ``SeriesGroupBy`` objects.
 

mars/dataframe/groupby/__init__.py

@@ -25,6 +25,7 @@ def _install():
     from .getitem import df_groupby_getitem
     from .head import head
     from .transform import groupby_transform
+    from .sample import groupby_sample
 
     for cls in DATAFRAME_TYPE:
         setattr(cls, 'groupby', groupby)
@@ -63,6 +64,8 @@ def _install():
 
         setattr(cls, 'head', head)
 
+        setattr(cls, 'sample', groupby_sample)
+
     for cls in DATAFRAME_GROUPBY_TYPE:
         setattr(cls, '__getitem__', df_groupby_getitem)