[wasm] Add SIMD detection.

[annxingyuan]

Adds a flag to evaluate SIMD support.

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This change is 

[lina128]

LGTM!

[tafsiri]

LGTM with one requested change (adding a generic async flag test in core).

Reviewed 4 of 4 files at r1.
Reviewable status: complete! 2 of 1 approvals obtained (waiting on @annxingyuan, @dsmilkov, @nsthorat, @pyu10055, and @tafsiri)

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tfjs-backend-wasm/src/index_test.ts, line 142 at r1 (raw file):

  });

  it('evaluating async flag works', async () => {

These look good, but when we talked I was thinking you could add a test in core to test a generic async flag. That way whether this code goes away or not, the code paths introduced in core get tested.

[annxingyuan]

Reviewable status: complete! 2 of 1 approvals obtained (waiting on @dsmilkov, @nsthorat, @pyu10055, and @tafsiri)

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tfjs-backend-wasm/src/index_test.ts, line 142 at r1 (raw file):

Previously, tafsiri (Yannick Assogba) wrote…

These look good, but when we talked I was thinking you could add a test in core to test a generic async flag. That way whether this code goes away or not, the code paths introduced in core get tested.

Done