Add docstrings to new HUB functions

[glenn-jocher]

@kalenmike @hassaanfarooq01 this PR adds single-line docstrings to the newly introduced functions that are lacking them.

Every function within our open-source repositories must be accompanied by at least a single-line docstring. These will also render within the Docs reference section in addition to the code.

EDIT: Also note that a lines that start with # are not actually docstrings and will be ignored by MkDocs and other tools. Instead the docstring, even if it's single-line, must start and end with """

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improving documentation and code clarity for model handling in the Ultralytics Hub.

📊 Key Changes

• Updated documentation strings for load_model, create_model, _set_train_args, _parse_identifier, retry_request, and _should_retry functions.
• Clarified the purpose of methods with more descriptive docstrings.

🎯 Purpose & Impact

• 📖 Improves code readability and documentation, making it easier for developers to understand the functionality of methods.
• 🛠️ Enhances maintainability of the codebase, as better commented code aids future updates and bug fixes.
• ☑️ Assists new developers or users in usability by clearly explaining what each function does and when it's called, potentially attracting a wider user base.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d762496) 76.13% compared to head (b5a5598) 39.60%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #7576       +/-   ##
===========================================
- Coverage   76.13%   39.60%   -36.54%     
===========================================
  Files         117      117               
  Lines       14328    14328               
===========================================
- Hits        10909     5674     -5235     
- Misses       3419     8654     +5235     

Flag	Coverage Δ
Benchmarks	?
GPU	39.60% <ø> (-0.02%)	⬇️
Tests	?

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

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