Add kernel RaggedGather for CPU and WebGL backend

[ahmedsabie]

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This change is 

[pyu10055]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @ahmedsabie and @lina128)

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tfjs-backend-cpu/src/kernels/RaggedGather_impl.ts line 53 at r1 (raw file):

    for (let i = 1; i < splits.length; ++i) {
      if (splits[i - 1] > splits[i]) {
        throw new Error('Ragged splits must be sorted');

in ascending order.

[pyu10055]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @ahmedsabie and @lina128)

a discussion (no related file):
$ tslint -p tsconfig_tslint.json
/workspace/tfjs-core/src/kernel_names.ts:677:1
ERROR: 677:1 no-consecutive-blank-lines Consecutive blank lines are forbidden

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tfjs-core/src/kernel_names.ts line 676 at r1 (raw file):

  rowPartitionTypes: string[];
}

extra empty line.

[lina128]

LGTM

Reviewable status: complete! 2 of 1 approvals obtained (waiting on @ahmedsabie)