Handle operators with no input (eg Constant)

[owulveryck]

Is your feature request related to a problem? Please describe.
The #116 raises an error because some operators, such as Constant do not have any input

Describe the solution you'd like
While decoding the graph in the core package, if an operator does not have any input, create a dummy node that will hold the backend of the current operator (same type, same shape).
It is the responsibility of the backend to reuse the data of the node to spare some memory.

Describe alternatives you've considered
For the constant operator, the operator could be turned into an initializer; but the problem would remain for other operators such as RandomUniform and RandomNormal

Additional context
cf Issue 2274 in the core onnx project.

[owulveryck]

branch #131 will follow the implementation

[owulveryck]

By now, the branch contains a basic implementation and a test. The implementation is creating a new ValueInfoProto when a node with zero input is detected while parsing.

@mattn maybe you can use this branch and merge your PR in it to see if it works?

[owulveryck]

All fixed and merged