[Misc] Create SamplingGraph Class (#6394)

Co-authored-by: Ubuntu <ubuntu@ip-172-31-39-125.ap-northeast-1.compute.internal>
Co-authored-by: Hongzhi (Steve), Chen <chenhongzhi.nkcs@gmail.com>

python/dgl/graphbolt/sampling_graph.py

+"""Sampling Graphs."""
+
+from typing import Dict, Union
+
+
+class SamplingGraph:
+    r"""Class for sampling graph."""
+
+    def __init__(self):
+        pass
+
+    @property
+    def num_nodes(self) -> Union[int, Dict[str, int]]:
+        """The number of nodes in the graph.
+        - If the graph is homogenous, returns an integer.
+        - If the graph is heterogenous, returns a dictionary.
+
+        Returns
+        -------
+        Union[int, Dict[str, int]]
+            The number of nodes. Integer indicates the total nodes number of a
+            homogenous graph; dict indicates nodes number per node types of a
+            heterogenous graph.
+        """
+        raise NotImplementedError
+
+    @property
+    def num_edges(self) -> Union[int, Dict[str, int]]:
+        """The number of edges in the graph.
+        - If the graph is homogenous, returns an integer.
+        - If the graph is heterogenous, returns a dictionary.
+
+        Returns
+        -------
+        Union[int, Dict[str, int]]
+            The number of edges. Integer indicates the total edges number of a
+            homogenous graph; dict indicates edges number per edge types of a
+            heterogenous graph.
+        """
+        raise NotImplementedError