[Enhancement] Add DGLGraph.to for PyTorch and MXNet backend (dmlc#600)

* add graph_to

* use backend copy_to

* add test

* fix test

* framework agnostic to() test

* disable pylint complaint

* add examples

* fix docstring

* formatting

* Format

* Update test_to_device.py

docs/source/api/python/graph.rst

@@ -79,6 +79,7 @@ Converting from/to other format
     DGLGraph.adjacency_matrix
     DGLGraph.adjacency_matrix_scipy
     DGLGraph.incidence_matrix
+    DGLGraph.to
 
 Using Node/edge features
 ------------------------

python/dgl/backend/pytorch/tensor.py

@@ -81,7 +81,8 @@ def copy_to(input, ctx):
     if ctx.type == 'cpu':
         return input.cpu()
     elif ctx.type == 'cuda':
-        th.cuda.set_device(ctx.index)
+        if ctx.index is not None:
+            th.cuda.set_device(ctx.index)
         return input.cuda()
     else:
         raise RuntimeError('Invalid context', ctx)

python/dgl/graph.py

@@ -3229,3 +3229,28 @@ def __repr__(self):
         return ret.format(node=self.number_of_nodes(), edge=self.number_of_edges(),
                           ndata=str(self.node_attr_schemes()),
                           edata=str(self.edge_attr_schemes()))
+
+    # pylint: disable=invalid-name
+    def to(self, ctx):
+        """
+        Move both ndata and edata to the targeted mode (cpu/gpu)
+        Framework agnostic
+
+        Parameters
+        ----------
+        ctx : framework specific context object
+
+        Examples (Pytorch & MXNet)
+        --------
+        >>> import backend as F
+        >>> G = dgl.DGLGraph()
+        >>> G.add_nodes(5, {'h': torch.ones((5, 2))})
+        >>> G.add_edges([0, 1], [1, 2], {'m' : torch.ones((2, 2))})
+        >>> G.add_edges([0, 1], [1, 2], {'m' : torch.ones((2, 2))})
+        >>> G.to(F.cuda())
+
+        """
+        for k in self.ndata.keys():
+            self.ndata[k] = F.copy_to(self.ndata[k], ctx)
+        for k in self.edata.keys():
+            self.edata[k] = F.copy_to(self.edata[k], ctx)

tests/backend/backend_unittest.py

@@ -9,6 +9,10 @@ def cuda():
     """Context object for CUDA."""
     pass
 
+def is_cuda_available():
+    """Check whether CUDA is available."""
+    pass
+
 ###############################################################################
 # Tensor functions on feature data
 # --------------------------------

tests/backend/mxnet/__init__.py

@@ -8,6 +8,14 @@
 def cuda():
     return mx.gpu()
 
+def is_cuda_available():
+    # TODO: Does MXNet have a convenient function to test GPU availability/compilation?
+    try:
+        a = nd.array([1, 2, 3], ctx=mx.gpu())
+        return True
+    except mx.MXNetError:
+        return False
+
 def array_equal(a, b):
     return nd.equal(a, b).asnumpy().all()
 

tests/backend/pytorch/__init__.py

@@ -5,6 +5,9 @@
 def cuda():
     return th.device('cuda:0')
 
+def is_cuda_available():
+    return th.cuda.is_available()
+
 def array_equal(a, b):
     return th.equal(a.cpu(), b.cpu())
 

tests/compute/test_to_device.py

@@ -0,0 +1,13 @@
+import dgl
+import backend as F
+
+def test_to_device():
+    g = dgl.DGLGraph()
+    g.add_nodes(5, {'h' : F.ones((5, 2))})
+    g.add_edges([0, 1], [1, 2], {'m' : F.ones((2, 2))})
+    if F.is_cuda_available():
+        g.to(F.cuda())
+
+
+if __name__ == '__main__':
+    test_to_device()