Optimize numberOfItems and numberOfSections in snapshot

[dtorres]

This avoids creating the intermediate collections to get these values

Checklist

• [x] All tests are passed.
• [ ] Added tests.
• [ ] Documented the code using Xcode markup.
• [x] Searched existing pull requests for ensure not duplicated.

Description

Related Issue

Motivation and Context

Impact on Existing Code

Screenshots (if appropriate)