Add support of SelectItem in ONNX-Chainer

[tkanmae]

This Pull Request addresses #8449. The implementation encodes an equivalent of the following NumPy snippet:

row_idxs = np.nonzero(np.ones(t.shape))[0]
n_cols = x.shape[1]
idxs = row_idxs * n_cols + t
output = np.ravel(x)[idxs]

I could have used Range to create row_idxs, but found that some of other ONNX conversion tools out there have not supported it yet. So I settled on the sequence of ConstantOfShape followed by NonZero to create the indices.

I'm not sure if I correctly used support decorator. Among the ops used in the implementation, ConstantOfShape is the newest one introduced in opset 9, and some ops have been updated in opset 11. So I set the argument to the decorator as (9, 11). It will be great if you can take a look at it.

[disktnk]

The alternative logic of F.select_item looks smart! SGTM,

So I set the argument to the decorator as (9, 11). It will be great if you can take a look at it

In this case, support decorator does not have to be set, but I agree to better looking.

[disktnk]

Could you add the comment of "the process is equivalent to get range"

[disktnk]

Could you split this test from TestArrayOperators, because this function is for unary args.

class TestSelectItem(ONNXModelTest):

    def test_output(self):

        class Model(chainer.Chain):

            def forward(self, x, t):
                return F.select_item(x, t)

        model = Model()
        x = input_generator.increasing(3, 3)
        t = np.array([2, 1, 0], dtype=np.int32)

        self.expect(
            model, (x, t), expected_num_initializers=0,
            skip_opset_version=(7, 8))

• set t as input, not to put unnecessary initializer
• set skip opset version

[disktnk]

@tkanmae Do you have special reason not to use GatherElements added from opset11? select_item could be repesented as GatherElements(Transpose(x), Unsqueeze(t)) (+ Squeeze) maybe.

[tkanmae]

@disktnk The reason I did not use GatherElements is that CoreML 3.0 does not support it. Thanks for the comments above. I will make changes as you suggested, hopefully in a few days.

[disktnk]

CoreML 3.0 does not support it

I see. We'll make another PR to use GatherElement when opset_version >= 11, and this alternative logic is remained to support under opset_version < 11 environnment. thx

[tkanmae]

@disktnk

We'll make another PR to use GatherElement when opset_version >= 11, and this alternative logic is remained to support under opset_version < 11 environnment.

Got it.

I fixed the bug in testing select_item() conversion as you suggested, though I set skip_opset_version to [1, 2, 3, 4, 5, 6, 7, 8] because ConstantOfShape is available from opset 9. I confirmed that the test passes on my machine.

[disktnk]

LTGM!

[disktnk]

Jenkins, test this please!

[chainer-ci]

Jenkins CI test (for commit 12376f5, target branch master) succeeded!

[disktnk]

thx! The travis fail is unrelated to this PR, so merge this manually. (The error has already resolved by another PR).

[chainer-ci]

Jenkins CI test (for commit 12376f5, target branch master) succeeded!