Fix backpropagation order in optimization tutorial

[patrocinio]

Reorder optimizer.zero_grad(), loss.backward(), and optimizer.step() to match the recommended best practice documented in the tutorial.

Fixes #3507

Description

Checklist

• [ ] The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
• [ ] Only one issue is addressed in this pull request
• [ ] Labels from the issue that this PR is fixing are added to this pull request
• [ ] No unnecessary issues are included into this pull request.