allow composed relu6 with unknown dims

[guschmue]

this should help #7, at least mobilenetv2 converts with this.

[pengwa]

I checked Max/Min support broadcasting, so why we cannot use it directly?

[guschmue]

uh, true but only since opset8 ... I can add this for opset8.
But our runtime doesn't support this yet ... won't be able to test it.

[pengwa]

@guschmue I see. 囧😳

[guschmue]

added the code to make use of opset 8 if available. I'll test with opset 8 as soon the runtime supports it (soon). In the meantime there is no risk since we default to opset 7.