Add support for ":0" indexing of nodes in graphdef

Sometimes the generated graph defs specify the 0'th output explicitly.
Example:
node {
  name: "abc"
  input: "Placeholder_1:0"
}
node {
  name: "Placeholder_1"
  op: "Placeholder"
}
Node abc specifies that it wants the 0th output of the Placeholder_1 node.
However in single output nodes, the tensor name is same as node name. So
Placeholder_1:0 tensor cannot be found. The same is also true for the 0th output
of multiple output nodes (output tensor names will be node_name,node_name:1,..)

So while parsing the graph def we strip away any ":0" from the input names.