use setImmediate instead of requestAnimationFrame on Node.js

[ChALkeR]

Description

This makes await tf.nextFrame(); work on Node.js for better code interoperability.

It also does the expected thing of unblocking the event loop and giving the engine time to perform other tasks, though the name is a bit confusing.

The order of the check is important: IE and Edge also implement setImmediate, but all versions that do that also have requestAnimationFrame, so requestAnimationFrame is used on those.

Refs: setImmediate, Node.js Event Loop.

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Tags: FEATURE, BREAKING, BUG, PERF, DEV, DOC, SECURITY

For more info see: https://github.com/tensorflow/tfjs/blob/master/DEVELOPMENT.md

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This change is 

[ChALkeR]

As a side note, it might be a good thing to use requestIdleCallback instead of requestAnimationFrame on browsers that implement it (currently FF and Chrome), but that is not included here.

[dsmilkov]

Reviewed 1 of 1 files at r1.
Reviewable status: 0 of 1 approvals obtained (waiting on @ChALkeR)

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src/browser_util.ts, line 22 at r1 (raw file):

  ? requestAnimationFrame // Browsers
  : setImmediate; // Node.js

This is great! Can you add a simple unit test that calls tf.nextFrame() and asserts that it calls a method? Introduce a browser_util_test.ts and use describeWithFlags('browser_util', ALL_ENVS, () => {}) which is a custom describe that runs on all envs.

[ChALkeR]

@dsmilkov I added tests, but using describeWithFlags('nextFrame' — that looks to be more in line with the other tests.

[dsmilkov]

Even better. Thanks!

Reviewed 1 of 1 files at r2.
Reviewable status: complete! 1 of 1 approvals obtained