Use a more robust algorithm for uniform random variates

[kgryte]

Description

This PR makes a small modification to the randUniform utility function, which is used when creating a tensor initialized with uniform random variates and during testing.

When a and b are approximately equal, the current algorithm is susceptible to catastrophic cancellation (a truncation of significant digits).

This PR modifies the computation of the random variate by replacing the subtraction of a and b with two multiplications, thus scaling the two input values.

The rearrangement of terms is found elsewhere, such as GSL, etc.

While the PR does add an additional operation (a multiplication), the effect on performance should be negligible on modern hardware and when run in JavaScript (as compared to, e.g., C or Assembly).

---

For repository owners only:

Please remember to apply all applicable tags to your pull request.
Tags: FEATURE, BREAKING, BUG, PERF, DEV, DOC, SECURITY

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

---

This change is 

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

[kgryte]

I signed the CLA.

[googlebot]

CLAs look good, thanks!

[dsmilkov]

Thank you for the amazing contribution!

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

[kgryte]

@dsmilkov No problem!