[wasm] Add StridedSlice kernel.

[annxingyuan]

Adds modularized StridedSlice kernel to WASM backend. This makes it possible to create a modularized WASM build for the blazeface model.

There is still a lot of logic duplicated between the new kernel and the forwardFunc in the op. I moved a little of it out into slice_util, but I thought it might be best to hold off on the rest until we modularize StridedSlice for a second backend.

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This change is 

[lina128]

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

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tfjs-backend-wasm/src/kernels/StridedSlice.ts, line 3 at r1 (raw file):

/**
 * @license
 * Copyright 2019 Google LLC. All Rights Reserved.

2020

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tfjs-backend-wasm/src/kernels/StridedSlice.ts, line 114 at r1 (raw file):

  }

  const out = backend.makeOutput(outShape, 'float32');

Hi Ann, just curious, why the large chunk of code is in JS but not in c++?

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tfjs-core/src/backends/backend_util.ts, line 40 at r1 (raw file):

import * as slice_util from '../ops/slice_util';
export {slice_util};

Why not export * from '../ops/slice_util' Is there naming conflict?

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tfjs-core/src/ops/strided_slice.ts, line 67 at r1 (raw file):

f (strides == null) {
    strides = new Array(begin.length);
  }

This part of the code should be in forward.

[annxingyuan]

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

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tfjs-backend-wasm/src/kernels/StridedSlice.ts, line 3 at r1 (raw file):

Previously, lina128 (Na Li) wrote…

2020

Done.

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tfjs-backend-wasm/src/kernels/StridedSlice.ts, line 114 at r1 (raw file):

Previously, lina128 (Na Li) wrote…

Hi Ann, just curious, why the large chunk of code is in JS but not in c++?

The chunk of code above normalizing the begin / end axes? It's cheap to run (despite its length), so keeping it in JS will allow it to be shared between cpu/webgl/wasm kernels.

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tfjs-core/src/backends/backend_util.ts, line 40 at r1 (raw file):

Previously, lina128 (Na Li) wrote…

Why not export * from '../ops/slice_util' Is there naming conflict?

Yes, there's a naming conflict with computeOutShape.

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tfjs-core/src/ops/strided_slice.ts, line 67 at r1 (raw file):

Previously, lina128 (Na Li) wrote…

f (strides == null) {
    strides = new Array(begin.length);
  }

This part of the code should be in forward.

Done.