[WASM] Add Slice and Square kernels

[dsmilkov]

Add Slice and Square kernels.

We do the slice in JS instead of C++ since we are copying memory around, which should be fast in JS. Also I added fast rank-specific implementations for tensors up to 4D, and a generic slow implementation for rank >= 5D.

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This change is 

[nsthorat]

Reviewed 8 of 8 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @annxingyuan, @dsmilkov, and @nsthorat)

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tfjs-backend-wasm/src/backend_wasm.ts, line 120 at r1 (raw file):

  memory() {
    return {
      // Unreliable due to automatic gc. The numbers above are cumulative.

I think this isn't true -- we malloc everything so this is identical to webgl. Unless slice semantics change this?

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tfjs-backend-wasm/src/cc/kernels/Square.cc, line 23 at r1 (raw file):

namespace {
inline float oper(float val) { return val * val; }

s/oper/square

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tfjs-core/src/ops/slice.ts, line 185 at r1 (raw file):

export const slice3d = op({slice3d_});
export const slice4d = op({slice4d_});
export {slice_util};

i dont think this is meant to be exported. If so, let's export it directly in index.ts rather than transitively from here.

[dsmilkov]

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

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tfjs-backend-wasm/src/backend_wasm.ts, line 120 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

I think this isn't true -- we malloc everything so this is identical to webgl. Unless slice semantics change this?

Oh yeah you are right. It is reliable. Also there is no distinction here like in webgl where we differentiate between numBytes and numBytesOnGPU since everything is on cpu all the time (no lazy upload)

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tfjs-backend-wasm/src/cc/kernels/Square.cc, line 23 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

s/oper/square

Done.

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tfjs-core/src/ops/slice.ts, line 185 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

i dont think this is meant to be exported. If so, let's export it directly in index.ts rather than transitively from here.

I need these utils to do slicing. Exported it top level.