Add unstack op

[Lewuathe]

Purpose

Some model including MobileNet requires unstack op (tensorflow/tfjs#188). Since unpack is depracated and renamed to unstack, TensorFlow.js should also use the same name convention.

see: tf.unstack

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This change is 

[nsthorat]

Thanks!

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Review status: 0 of 4 files reviewed at latest revision, all discussions resolved.

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src/ops/array_ops.ts, line 1031 at r1 (raw file):

  }

  static unstack<T extends Tensor>(

Can you add a jsdoc here with a code snippet? Put this along side stack in docs

Also add @operation so we clean up intermediates.

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src/ops/array_ops_test.ts, line 2113 at r1 (raw file):

  });
});

can you add one unit tests with num = 3, ideally with

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src/ops/array_ops_test.ts, line 2129 at r1 (raw file):

  it('unstack by axis=1', () => {
    const x = tf.tensor2d([1, 2, 3, 4, 5, 6, 7, 8], [2, 4]);
    const res = tf.unstack(x, 4, 1);

can you assert the other values? res[1], res[2] etc and their rank and shape? Here and elsewhere

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Comments from Reviewable

[dsmilkov]

Nice work. Sorry for the delay. Nikhil and I were traveling last week.

I noticed you addressed Nikhil's comments. I also left couple comments, one about removing the num parameter since in TF.js the shape is always known.

Not actionable in this PR: Having unstack implemented using slice is fine for now but we should consider adding this as a backend method and implement it separately for webgl and cpu, and nodejs, especially since Tensorflow has it as a separate kernel (unpack which we can call in our nodejs backend directly.

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Reviewed 2 of 4 files at r1, 2 of 2 files at r2.
Review status: all files reviewed at latest revision, 3 unresolved discussions, some commit checks failed.

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src/ops/array_ops.ts, line 1040 at r2 (raw file):

   *
   * @param value A tensor object.
   * @param num The number of unstacked tensors.

can you remove the num parameter since in TF.js the shape is always known (the engine is eager mode)?

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src/ops/array_ops.ts, line 1056 at r2 (raw file):

    const outputShape: number[]
        = Array(value.rank - 1).fill(0);

the 2 lines above can fit in a single line

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Comments from Reviewable

[dsmilkov]

Thank you Kai!

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Reviewed 4 of 4 files at r3.
Review status: all files reviewed at latest revision, 5 unresolved discussions, some commit checks failed.

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Comments from Reviewable