Added ArgMax operator

[Swopper050]

Closes #14
Closes #210

[Swopper050]

@wipsel What should we do about the string constant lint error?

The attributes 'axis' and 'keepdims' occur in (at least) 3 operators, causing the linter to say it should be a constant. However, it does not really feel nice to group some of the attributes together and some not (like select_last_index in this operator). Do you have an idea how we could do this?

[wipsel]

@wipsel What should we do about the string constant lint error?

The attributes 'axis' and 'keepdims' occur in (at least) 3 operators, causing the linter to say it should be a constant. However, it does not really feel nice to group some of the attributes together and some not (like select_last_index in this operator). Do you have an idea how we could do this?

I see two options. Make all attribute names a constant in their operator. That would make the linter happy and i don't really see a problem with it. It could be a nice way to add documentation about the attribute if need be.

Another option would be to create an attrs or attrname package defining the attribute names of all operators "globally"
The usage would than look something like this:

// in attrs/const.go
const (
    KeepDims = "keep_dims"
    Axis = "axis"
)

// in some operator 
import attrs


// Init initializes the argmax operator.
func (a *ArgMax) Init(n *onnx.NodeProto) error {
	attributes := n.GetAttribute()
	for _, attr := range attributes {
		switch attr.GetName() {
		case attrs.Axis:
                     // do  attr stuff
               }
       } 
}

I don't know how much of the attr names are shared and if it justifies a seperate package. Maybe you have a better view on this. If not you could define them under their own name under their own operator. I would however make it consistent with all the attr names for the same operator. So if you change them to a const for this i would include the select_last_index as wel. Don't know if it all makes sense but hope it helps

[Swopper050]

I see two options. Make all attribute names a constant in their operator. That would make the linter happy and i don't really see a problem with it. It could be a nice way to add documentation about the attribute if need be.

Let's do this. I created an issue #211 for it and for this MR only fixed it in ArgMax.