Gorgonia backend does not handle model with several root nodes

[mattn]

Is your feature request related to a problem? Please describe.

I'm trying to run ssd using onnx-go. But error occured.

https://gist.github.com/mattn/f5aa1b96753c76075f2666235919a8f3

2019/05/09 11:09:55 onnx: operator  not implemented ()

Describe the solution you'd like

As far as I looked code backend/x/gorgonnx, the error seems occur at here:

onnx-go/backend/x/gorgonnx/graph.go

Lines 59 to 61 in 2f00eed

	if len(g.roots) != 1 {
	return &onnx.ErrNotImplemented{}
	}

Describe alternatives you've considered

Sorry, I can't figure out what should do here.

[owulveryck]

Good catch!

The model has multiple "root" nodes which is not handled yet by the backend. This is an extract of the model rendered with netron:

Two things to do:

• setting a more explicit error message
• implement the feature

I will split this into two issues.

[owulveryck]

I am changing the title of this issue; I have created a "project" to follow the implementation steps to make the SSD model working.

Note: I have analyzed the model, and it will require a lot of effort to make it work with gorgonia.

[owulveryck]

There is a work-in-progress with the branch issue-48 but it needs some more investigation.

The actual design of "gorgonnx" does not handle operators with multiple output.

the signature of the apply method will probably require some changes:

type operator interface {
        // apply analyse the graph to find the children of the node
        // then extract its gorgonia.Node references
        // and assign the result of the operation to the node n
        apply(g *Graph, n *Node) error
        // init the operator with name and attributes as carried by the onnx.Operator
        init(o onnx.Operation) error
}

[owulveryck]

PR #115 fixes this error, but it introduces a breaking change.
The OperationCarrier interface needed to be changed to handle nodes with multiple outputs.

onnx-go/backend.go

Lines 19 to 27 in f9baf80

	// OperationCarrier should be a method of the graph
	// because the operation needs the topology of the graph
	// to check the arity of the node for example
	type OperationCarrier interface {
	// ApplyOperation on the graph nodes
	// graph.Node is an array because it allows to handle multiple output
	// for example a split operation returns n nodes...
	ApplyOperation(Operation, ...graph.Node) error
	}

This will require to change any backend implementation.
As far as I know, the only backend implemented so far is Gorgonia (via gorgonnx), and it has been patched.