Minor updates to optimizer initializers

[saitcakmak]

Summary:

• Simplified a mini-batch evaluation step, which seemed quite complex when done manually.
• Updated a few type hints.
• Added stacklevel arg to some warnings.

Differential Revision: D64267268

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D64267268

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.98%. Comparing base (c48db81) to head (b523da5). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2570 +/- ## ========================================== - Coverage 99.98% 99.98% -0.01% ========================================== Files 195 195 Lines 17122 17116 -6 ========================================== - Hits 17119 17113 -6 Misses 3 3 ```

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[facebook-github-bot]

This pull request has been merged in pytorch/botorch@85d89962e09cac32dc30dce13e379cd8a85c24e0.