[MLBuffer] Support for MLBuffer in graph execution

[bbernhar]

Purpose/Motivation

Provides a means to execute a MLGraph using MLBuffer. This is a sub-issue of #482.

Proposed API

dictionary MLBufferView {
  required MLBuffer buffer;
  MLSize64 offset = 0;
  MLSize64 size;
};

typedef record<DOMString, MLBufferView> MLNamedMLBufferViews;

[Exposed=(Window, DedicatedWorker), SecureContext]
partial interface MLContext {
  undefined dispatch(
        MLGraph graph, MLNamedMLBufferViews inputs, MLNamedMLBufferViews outputs);
}

Example JS

const bufferA = new MLBuffer({size:4});
const bufferB = new MLBuffer({size:4});
const inputs = {'A': {bufferA, 0, bufferA.size()};
const outputs = {'B': {bufferB, 0, bufferB.size()};
context.dispatch(graph, inputs, outputs);

• Enqueues a request to compute the graph onto some WebNN timeline
• Execution cannot start until all input and output MLBuffers are available
• All input and output MLBuffers are unavailable while execution is in progress
• All work submitted after this dispatch() call which relies on an input or output MLBuffer will be queued behind this execution

Alternative API proposals

N/A

Opens

1. Should this method be on the MLGraph (related to API simplification: context owns builder, graph becomes internal slot #303)? @a-sully
2. Is it valid to pass the same MLBuffer as both an input and output of the same dispatch() call? @a-sully
3. If the approach is flexible enough to allow for graph execution on all backends. Do we need a separate compute() method? @a-sully
4. Can dispatch be exclusive to MLBuffer bindings? @bbernhar