Add object detection API for AutoML

[dsmilkov]

• Add pre and post-process ops for the model (fast non-max-suppression)
• Add the API for loading and running on object detection model trained by AutoML
• Add unit tests for both node and the browser environment using a real model produced by AutoML

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This change is 

[nsthorat]

Reviewed 20 of 20 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @dsmilkov and @nsthorat)

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tfjs-automl/src/object_detection.ts, line 91 at r1 (raw file):

    // Run post process in cpu for speed.
    const prevBackend = getBackend();
    setBackend('cpu');

as we spoke about in person: remove this

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tfjs-automl/src/object_detection.ts, line 96 at r1 (raw file):

        boxesTensor as Tensor2D, boxScores, options.topk, options.iou,
        options.score);
    const selectedBoxes = selectedBoxesTensor.dataSync() as Int32Array;

.data()

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tfjs-automl/src/object_detection.ts, line 164 at r1 (raw file):

    const boxIndex = selectedBoxes[i];
    const [top, left, bottom, right] =
        Array.from(boxes.slice(boxIndex * 4, boxIndex * 4 + 4));

maybe pull 4 to a constant so it's clear why it's 4

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tfjs-automl/src/object_detection_test.ts, line 48 at r1 (raw file):

    const options = {score: 0.11};
    const predictions = await model.detect(img, options);
    const {box: {left, top, width, height}, label, score} = predictions[0];

assert predictions.length = 1

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tfjs-automl/src/util.ts, line 26 at r1 (raw file):

/** Loads and parses the dictionary. */
export async function loadDictionary(modelUrl: string): Promise<string[]> {

might be worth testing this

[dsmilkov]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @nsthorat)

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tfjs-automl/src/object_detection.ts, line 91 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

as we spoke about in person: remove this

Done.

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tfjs-automl/src/object_detection.ts, line 96 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

.data()

Done.

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tfjs-automl/src/object_detection.ts, line 164 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

maybe pull 4 to a constant so it's clear why it's 4

Done.

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tfjs-automl/src/object_detection_test.ts, line 48 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

assert predictions.length = 1

Done.

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tfjs-automl/src/util.ts, line 26 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

might be worth testing this

I test it via the public API model.dictionary for both models I load.