Add tf.spaceToBatchND

[dikatok]

Description

Adds batchToSpaceND to the API tensorflow/tfjs#161.

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For more info see: https://github.com/tensorflow/tfjs/blob/master/DEVELOPMENT.md

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This change is 

[dikatok]

@rodrigob the only interaction I can think of is using them to implement atrous convolution, or do you have any suggestions?

[dsmilkov]

This is really nice and it's almost there! Left 3 small comments. Ping me when you address it and I'll be happy to merge!

Reviewed 7 of 7 files at r1.
Reviewable status: 0 of 1 approvals obtained (waiting on @dikatok)

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src/kernels/backend.ts, line 205 at r1 (raw file):

  spaceToBatchND<T extends Tensor>(
      x: T, blockShape: number[], crops: number[][]): T;

rename crops to paddings for consistency

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src/ops/array_ops.ts, line 810 at r1 (raw file):

}

function spaceToBatchND_<T extends Tensor>(

Add a jsdoc to this method similar to the one for batchToSpaceND above. Also make sure to include the @doc {heading: 'Tensors', subheading: 'Transformations'} tag so we can surface the method in our website api page: https://js.tensorflow.org/api/latest

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src/ops/array_ops_test.ts, line 3206 at r1 (raw file):

  });

  it('tensor4d, input shape=[1, 4, 4, 1], blockShape=[2, 2]', () => {

add you add one more test with input shape [2, 6, 6, 1] and block shape [2, 2] and verify its output is [8, 3, 3, 1] (used 6 with 2 to break the symmetry: 6 = 2 * 3, since 4 = 2 * 2 makes the block size equal the leftover spacial size)

[rodrigob]

@dikatok I have not studied the operations definition in detail, but is there not an expected symmetry ?

Something like
assert A == spaceToBatchND( batchToSpaceND( A ) ) ?

[dikatok]

@rodrigob hmmm ok I will add some symmetry tests

[dsmilkov]

@rodrigob That's a great suggestion

[dikatok]

whoops, forgot to ping you @dsmilkov

[dsmilkov]

Really nice work! Thank you!

Reviewed 3 of 3 files at r3.
Reviewable status: 0 of 1 approvals obtained