[Model] add implementation of twirls (#2770)

* add implementation of twirls

* format the code

* fix some format error, and ignore others

* fix format errors

* fix format errors

* expose unfolding & attention

* Update nn.pytorch.rst
Co-authored-by: Quan (Andy) Gan <coin2028@hotmail.com>

docs/source/api/python/nn.pytorch.rst

@@ -136,6 +136,20 @@ DotGatConv
     :members: forward
     :show-inheritance:
 
+TWIRLSConv
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: dgl.nn.pytorch.conv.TWIRLSConv
+    :members: forward
+    :show-inheritance:
+
+TWIRLSUnfoldingAndAttention
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: dgl.nn.pytorch.conv.TWIRLSUnfoldingAndAttention
+    :members: forward
+    :show-inheritance:
+
 .. _apinn-pytorch-dense-conv:
 
 Dense Conv Layers

python/dgl/nn/pytorch/conv/__init__.py

@@ -21,8 +21,10 @@ from .densesageconv import DenseSAGEConv
 from .atomicconv import AtomicConv
 from .cfconv import CFConv
 from .dotgatconv import DotGatConv
+from .twirlsconv import TWIRLSConv, UnfoldingAndAttention as TWIRLSUnfoldingAndAttention
 
 __all__ = ['GraphConv', 'EdgeWeightNorm', 'GATConv', 'TAGConv', 'RelGraphConv', 'SAGEConv',
            'SGConv', 'APPNPConv', 'GINConv', 'GatedGraphConv', 'GMMConv',
            'ChebConv', 'AGNNConv', 'NNConv', 'DenseGraphConv', 'DenseSAGEConv',
-           'DenseChebConv', 'EdgeConv', 'AtomicConv', 'CFConv', 'DotGatConv']
+           'DenseChebConv', 'EdgeConv', 'AtomicConv', 'CFConv', 'DotGatConv', 'TWIRLSConv',
+           'TWIRLSUnfoldingAndAttention']