Fix issue #213 (#221)

Dropout layers were not handled properly in SummaryGraph, and
caused the indexing of layer names to change.
The root cause is that in ONNX uses the same node name for
Dropout and Linear layers that are processed in sequence.
ONNX nodes can be identified by three components: the ONNX 
node name,  type, and instance.
In SummaryGraph we ignore the node type when naming a node.
Specifically in AlexNet, nodes the Dropout layers before a Linear
layer have the same node name and instance, and are only distinguished
by their type.  SummaryGraph, ignorant of the type, skipped the Dropout
layers and gave SG nodes the wrong name.  Thus 'classifier.0', which is
a Dropout node, became a Linear node.
The fix is not to ignore duplicate (node name, instance) pairs
by incrementing the instance.