[Utils] Edge and LINKX homophily measure (#5382)

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* edge homophily

* linkx homophily

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---------
Co-authored-by: Ubuntu <ubuntu@ip-172-31-36-188.ap-northeast-1.compute.internal>

docs/source/api/python/dgl.rst

@@ -212,7 +212,9 @@ Utilities for measuring homophily of a graph
 .. autosummary::
     :toctree: ../../generated/
 
+    edge_homophily
     node_homophily
+    linkx_homophily
 
 Utilities
 -----------------------------------------------

python/dgl/homophily.py

 """Utils for tacking graph homophily and heterophily"""
-from . import backend as F, function as fn
+# pylint: disable=W0611
+from . import function as fn
 
-__all__ = ["node_homophily"]
+try:
+    import torch
+except ImportError:
+    HAS_TORCH = False
+else:
+    HAS_TORCH = True
+
+__all__ = ["node_homophily", "edge_homophily", "linkx_homophily"]
+
+
+def check_pytorch():
+    """Check if PyTorch is the backend."""
+    if HAS_TORCH is False:
+        raise ModuleNotFoundError(
+            "This function requires PyTorch to be the backend."
+        )
+
+
+def get_long_edges(graph):
+    """Internal function for getting the edges of a graph as long tensors."""
+    src, dst = graph.edges()
+    return src.long(), dst.long()
 
 
 def node_homophily(graph, y):
-    r"""Homophily measure from `Geom-GCN: Geometric Graph Convolutional Networks
-    <https://arxiv.org/abs/2002.05287>`__
+    r"""Homophily measure from `Geom-GCN: Geometric Graph Convolutional
+    Networks <https://arxiv.org/abs/2002.05287>`__
 
     We follow the practice of a later paper `Large Scale Learning on
     Non-Homophilous Graphs: New Benchmarks and Strong Simple Methods
@@ -15,8 +37,8 @@ def node_homophily(graph, y):
     Mathematically it is defined as follows:
 
     .. math::
-      \frac{1}{|\mathcal{V}|} \sum_{v \in \mathcal{V}} \frac{ | \{ (u,v) : u
-      \in \mathcal{N}(v) \wedge y_v = y_u \} |  } { |\mathcal{N}(v)| }
+      \frac{1}{|\mathcal{V}|} \sum_{v \in \mathcal{V}} \frac{ | \{u
+      \in \mathcal{N}(v): y_v = y_u \} |  } { |\mathcal{N}(v)| },
 
     where :math:`\mathcal{V}` is the set of nodes, :math:`\mathcal{N}(v)` is
     the predecessors of node :math:`v`, and :math:`y_v` is the class of node
@@ -25,14 +47,14 @@ def node_homophily(graph, y):
     Parameters
     ----------
     graph : DGLGraph
-        The graph
-    y : Tensor
-        The node labels, which is a tensor of shape (|V|)
+        The graph.
+    y : torch.Tensor
+        The node labels, which is a tensor of shape (|V|).
 
     Returns
     -------
     float
-        The node homophily value
+        The node homophily value.
 
     Examples
     --------
@@ -44,14 +66,124 @@ def node_homophily(graph, y):
     >>> dgl.node_homophily(graph, y)
     0.6000000238418579
     """
+    check_pytorch()
+    with graph.local_scope():
+        # Handle the case where graph is of dtype int32.
+        src, dst = get_long_edges(graph)
+        # Compute y_v = y_u for all edges.
+        graph.edata["same_class"] = (y[src] == y[dst]).float()
+        graph.update_all(
+            fn.copy_e("same_class", "m"), fn.mean("m", "same_class_deg")
+        )
+        return graph.ndata["same_class_deg"].mean(dim=0).item()
+
+
+def edge_homophily(graph, y):
+    r"""Homophily measure from `Beyond Homophily in Graph Neural Networks:
+    Current Limitations and Effective Designs
+    <https://arxiv.org/abs/2006.11468>`__
+
+    Mathematically it is defined as follows:
+
+    .. math::
+      \frac{| \{ (u,v) : (u,v) \in \mathcal{E} \wedge y_u = y_v \} | }
+      {|\mathcal{E}|},
+
+    where :math:`\mathcal{E}` is the set of edges, and :math:`y_u` is the class
+    of node :math:`u`.
+
+    Parameters
+    ----------
+    graph : DGLGraph
+        The graph.
+    y : torch.Tensor
+        The node labels, which is a tensor of shape (|V|).
+
+    Returns
+    -------
+    float
+        The edge homophily ratio value.
+
+    Examples
+    --------
+    >>> import dgl
+    >>> import torch
+
+    >>> graph = dgl.graph(([1, 2, 0, 4], [0, 1, 2, 3]))
+    >>> y = torch.tensor([0, 0, 0, 0, 1])
+    >>> dgl.edge_homophily(graph, y)
+    0.75
+    """
+    check_pytorch()
+    with graph.local_scope():
+        # Handle the case where graph is of dtype int32.
+        src, dst = get_long_edges(graph)
+        # Compute y_v = y_u for all edges.
+        edge_indicator = (y[src] == y[dst]).float()
+        return edge_indicator.mean(dim=0).item()
+
+
+def linkx_homophily(graph, y):
+    r"""Homophily measure from `Large Scale Learning on Non-Homophilous Graphs:
+    New Benchmarks and Strong Simple Methods
+    <https://arxiv.org/abs/2110.14446>`__
+
+    Mathematically it is defined as follows:
+
+    .. math::
+      \frac{1}{C-1} \sum_{k=1}^{C} \max \left(0, \frac{\sum_{v\in C_k}|\{u\in
+      \mathcal{N}(v): y_v = y_u \}|}{\sum_{v\in C_k}|\mathcal{N}(v)|} -
+      \frac{|\mathcal{C}_k|}{|\mathcal{V}|} \right),
+
+    where :math:`C` is the number of node classes, :math:`C_k` is the set of
+    nodes that belong to class k, :math:`\mathcal{N}(v)` are the predecessors
+    of node :math:`v`, :math:`y_v` is the class of node :math:`v`, and
+    :math:`\mathcal{V}` is the set of nodes.
+
+    Parameters
+    ----------
+    graph : DGLGraph
+        The graph.
+    y : torch.Tensor
+        The node labels, which is a tensor of shape (|V|).
+
+    Returns
+    -------
+    float
+        The homophily value.
+
+    Examples
+    --------
+    >>> import dgl
+    >>> import torch
+
+    >>> graph = dgl.graph(([0, 1, 2, 3], [1, 2, 0, 4]))
+    >>> y = torch.tensor([0, 0, 0, 0, 1])
+    >>> dgl.linkx_homophily(graph, y)
+    0.19999998807907104
+    """
+    check_pytorch()
     with graph.local_scope():
-        src, dst = graph.edges()
+        # Compute |{u\in N(v): y_v = y_u}| for each node v.
         # Handle the case where graph is of dtype int32.
-        src = F.astype(src, F.int64)
-        dst = F.astype(dst, F.int64)
+        src, dst = get_long_edges(graph)
         # Compute y_v = y_u for all edges.
-        graph.edata["same_class"] = F.astype(y[src] == y[dst], F.float32)
+        graph.edata["same_class"] = (y[src] == y[dst]).float()
         graph.update_all(
-            fn.copy_e("same_class", "m"), fn.mean("m", "node_value")
+            fn.copy_e("same_class", "m"), fn.sum("m", "same_class_deg")
         )
-        return graph.ndata["node_value"].mean().item()
+
+        deg = graph.in_degrees().float()
+        num_nodes = graph.num_nodes()
+        num_classes = y.max(dim=0).values.item() + 1
+
+        value = 0
+        for k in range(num_classes):
+            # Get the nodes that belong to class k.
+            class_mask = y == k
+            same_class_deg_k = graph.ndata["same_class_deg"][class_mask].sum()
+            deg_k = deg[class_mask].sum()
+            num_nodes_k = class_mask.sum()
+            value += max(0, same_class_deg_k / deg_k - num_nodes_k / num_nodes)
+
+        return value.item() / (num_classes - 1)

tests/python/common/test_homophily.py

+import math
 import unittest
 
 import backend as F
@@ -6,7 +7,9 @@ import dgl
 from test_utils import parametrize_idtype
 
 
-@unittest.skipIf(dgl.backend.backend_name == "tensorflow", reason="Skip TF")
+@unittest.skipIf(
+    dgl.backend.backend_name != "pytorch", reason="Only support PyTorch for now"
+)
 @parametrize_idtype
 def test_node_homophily(idtype):
     # IfChangeThenChange: python/dgl/homophily.py
@@ -16,4 +19,32 @@ def test_node_homophily(idtype):
         ([1, 2, 0, 4], [0, 1, 2, 3]), idtype=idtype, device=device
     )
     y = F.tensor([0, 0, 0, 0, 1])
-    assert dgl.node_homophily(graph, y) == 0.6000000238418579
+    assert math.isclose(dgl.node_homophily(graph, y), 0.6000000238418579)
+
+
+@unittest.skipIf(
+    dgl.backend.backend_name != "pytorch", reason="Only support PyTorch for now"
+)
+@parametrize_idtype
+def test_edge_homophily(idtype):
+    # IfChangeThenChange: python/dgl/homophily.py
+    # Update the docstring example.
+    device = F.ctx()
+    graph = dgl.graph(
+        ([1, 2, 0, 4], [0, 1, 2, 3]), idtype=idtype, device=device
+    )
+    y = F.tensor([0, 0, 0, 0, 1])
+    assert math.isclose(dgl.edge_homophily(graph, y), 0.75)
+
+
+@unittest.skipIf(
+    dgl.backend.backend_name != "pytorch", reason="Only support PyTorch for now"
+)
+@parametrize_idtype
+def test_linkx_homophily(idtype):
+    # IfChangeThenChange: python/dgl/homophily.py
+    # Update the docstring example.
+    device = F.ctx()
+    graph = dgl.graph(([0, 1, 2, 3], [1, 2, 0, 4]), device=device)
+    y = F.tensor([0, 0, 0, 0, 1])
+    assert math.isclose(dgl.linkx_homophily(graph, y), 0.19999998807907104)