Add tf.topk

[dsmilkov]

Add tf.topk.

This is required in ssd_mobilenet_v2: tensorflow/tfjs#188. Uses a shared implementation across webgl and cpu where the values are downloaded and sorted on the cpu. There is already an issue that tracks topk on the gpu, which we can revisit later: tensorflow/tfjs#94

FEATURE

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This change is 

[nsthorat]

Reviewed 9 of 9 files at r1.
Reviewable status: 0 of 1 approvals obtained (waiting on @dsmilkov and @nsthorat)

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src/ops/topk.ts, line 34 at r1 (raw file):

  if (k > lastDim) {
    throw new Error(
        `'k' passed to topk() must be <= the last dimension (${lastDim})`);

but got ${k}

[dsmilkov]

cc @pyu10055 for adding support in the converter after this is in.

[dsmilkov]

Reviewable status: complete! 1 of 1 approvals obtained

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src/ops/topk.ts, line 34 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

but got ${k}

Done