[wasm] Export version.

[annxingyuan]

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This change is 

[nsthorat]

Reviewed 1 of 1 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/index.ts, line 20 at r1 (raw file):

import './kernels/all_kernels';
export {BackendWasm, setWasmPath} from './backend_wasm';
export {version as version_wasm} from './version';

add a unit test for this like we do in other packages: https://github.com/tensorflow/tfjs/blob/master/tfjs-layers/src/version_test.ts