Improve test readability (Layer invoke operator)

[dosier]

I propose that we create something akin to this (but then as a function of Layer):

    private operator fun RepeatVector.invoke(input: Array<FloatArray>): Output<Float> {
        return Ops.create().let { tf ->
            build(tf, KGraph(Graph().toGraphDef()), Shape.make(10, 10))
            val inputOp = tf.constant(input)
            val isTraining = tf.constant(true)
            val numberOfLosses = tf.constant(1.0f)
            forward(tf, inputOp, isTraining, numberOfLosses).asOutput()
        }
    }

So that we can do this:

        val x = Array(3) { FloatArray(3) { it.toFloat() } }
        val y = layer(x)

Which also nicely matches tests in Keras (the invoke part).

[zaleslaw]

It's an interesting idea, @dosier , this looks like a special functionality for testing and debugging purposes only, and we have no such functionality. I'm not sure that we should have this in the public API because it creates a separate graph for the layer under the hood.

Currently, the @mkaze is working on the same thing but external to Layer in #103.

But this idea not bad, and probably we could create some invoke extension (like in your example) and make it internal in the tests? Extra-functionality for test purposes in the test module.

After some time of usage, it will be better to understand whether we should open it to the world via Layer API.