Add a __dataframe__ method to the protocol dataframe object

This will align the implementation with those in other libraries,
xref data-apis/dataframe-api#80.

Author: rgommers

python/cudf/cudf/core/df_protocol.py

@@ -529,6 +529,16 @@ def __init__(
         self._nan_as_null = nan_as_null
         self._allow_copy = allow_copy
 
+    def __dataframe__(
+        self, nan_as_null: bool = False, allow_copy: bool = True
+    ) -> "_CuDFDataFrame":
+        """
+        See the docstring of the `cudf.DataFrame.__dataframe__` for details
+        """
+        return _CuDFDataFrame(
+            self._df, nan_as_null=nan_as_null, allow_copy=allow_copy
+        )
+
     @property
     def metadata(self):
         # `index` isn't a regular column, and the protocol doesn't support row

python/cudf/cudf/tests/test_df_protocol.py

@@ -124,6 +124,9 @@ def test_from_dataframe():
     df2 = cudf.from_dataframe(df1)
     assert_eq(df1, df2)
 
+    df3 = cudf.from_dataframe(df2)
+    assert_eq(df1, df3)
+
 
 def test_int_dtype():
     data_int = dict(a=[1, 2, 3], b=[9, 10, 11])