[Python] Support max_batch_weight in BatchElements and honor element_size_fn in stateful batching

[tvalentyn]

• Add max_batch_weight to BatchElements. When set, the batch size limits
  count elements and element_size_fn gives the weight of an element. This
  fixes RunInference failing with a TypeError when a ModelHandler sets
  max_batch_weight.
• Honor element_size_fn when max_batch_duration_secs is set. This uses a
  separate stateful DoFn that only adds a 'weight' state and keeps the
  transform label, so pipelines that don't set element_size_fn are
  unaffected.
• Never emit an empty batch when an element exceeds the limits on its own.
• Warn when length_fn is set without max_batch_duration_secs, since it is
  ignored.<!--

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• If this contribution is large, please file an Apache Individual Contributor License Agreement.
• Replace these instructions with a description of your change and why you have made it.
• Mention the appropriate issue in your description (for example: addresses #123 or fixes #456).
• Update CHANGES.md with noteworthy changes.

See CONTRIBUTING.md for more details.

-->

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.92%. Comparing base (76f6e6d) to head (80e18e0).
⚠️ Report is 178 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #40603       +/-   ##
=============================================
+ Coverage     56.16%   79.92%   +23.75%     
=============================================
  Files          1124      488      -636     
  Lines        178336    82489    -95847     
  Branches       1489        0     -1489     
=============================================
- Hits         100170    65929    -34241     
+ Misses        75645    16560    -59085     
+ Partials       2521        0     -2521     

Flag	Coverage Δ
go	?
java	?
python	79.92% <100.00%> (+0.36%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.