use instantiateWasm override and platform fetch

[pyu10055]

This allows platforms like WeChat miniprogram to load the wasm file using
the platform fetch method.

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This change is 

[dsmilkov]

Thanks. Smallish comments. LGTM

Reviewed 5 of 5 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @dsmilkov and @pyu10055)

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tfjs-backend-wasm/README.md, line 90 at r1 (raw file):

```ts
import {setWasmPath} from '@tensorflow/tfjs-backend-wasm';
setWasmPath(yourCustomPath, true); // or tf.wasm.setWasmPath when using <script> tags.

for readability extract the true as a separate variable:

const usePlatformFetch = true;
setWasmPath(yourCustomPath, usePlatformFetch);

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tfjs-backend-wasm/src/backend_wasm.ts, line 213 at r1 (raw file):

      };
      if (customFetch) {
        factoryConfig.instantiateWasm = createInstantiateWasmFunc(wasmPath);

for reference, add a comment with the above link:
https://github.com/emscripten-core/emscripten/blob/2bca083cbbd5a4133db61fbd74d04f7feecfa907/tests/manual_wasm_instantiate.html#L170

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tfjs-backend-wasm/src/backend_wasm.ts, line 280 at r1 (raw file):

 * @param path wasm file path or url
 * @param usePlatformFetch optional boolean to use platform fetch to download
 *     the wasm file

append "Defaults to false." in the jsdoc

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tfjs-backend-wasm/src/backend_wasm.ts, line 283 at r1 (raw file):

 */
/** @doc {heading: 'Environment', namespace: 'wasm'} */
export function setWasmPath(path: string, usePlatformFetch?: boolean): void {

use explicit defaults in the function signature: ...usePlatformFetch = false)

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tfjs-backend-wasm/src/index_test.ts, line 84 at r1 (raw file):

  it('backend init fails when the path is invalid and use platform fetch',
     async () => {
       setWasmPath('invalid/path', true);

extract as a variable usePlatformFetch

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tfjs-backend-wasm/src/index_test.ts, line 95 at r1 (raw file):

     });

  it('backend init succeeds with default path', async () => {

can you add 1 more test, a VALID path with usePlatformFetch=true