As discussed in webmachinelearning#572:

- Define an "operator" concept, with inputs, outputs, activations.
- Defer "platform operator" and "platform operand" to build.
- Standardize the graph connection steps across builder methods.
- Simplify activation, input and constant steps.

Not covered in this change:

- Erroring if input's [[builder]] doesn't match `this`
- Build algorithm - covered by webmachinelearning#448 and webmachinelearning#457
- gru() is missing steps to populate output. Added "Issue" note.
- Introducing "Validate arguments" section for each method.
- Introducing "Calculate output shape" section for each method.

For webmachinelearning#549 and webmachinelearning#572.

…odel-operators

In tip-of-tree, the "desc" MLOperandDescriptor is populated, used,
then modified and conditionally used again (if `returnSequence` is
true) when creating "output2". This was broken in previous commits in
this branch. Restore the intent, using a separate "desc2"
MLOperandDescriptor only conditionally populated and then
conditionally used.

…odel-operators

…odel-operators

…odel-operators

Co-authored-by: Ningxin Hu <[email protected]>

…s' into content-programming-model-operators

* Add other operand inputs
* PreLU -> PReLU
* Define split() output shapes calculations