Fix ModelCheckpoint with manual optimization and every_n_train_steps

[littlebullGit]

This PR enhances callback to properly handle manual optimization scenarios, ensuring checkpoints reflect the intended model state and providing clear user guidance.

Fixes #20947

Key Changes:

• Manual Optimization Support: Ensures checkpoints capture the model state before optimization when using manual optimization with every_n_train_steps.
• User Warning: Adds a clear warning when pre-optimization state isn't saved, helping users understand the checkpoint behavior.
• Documentation: Updates docstrings and examples to clarify the behavior with manual optimization.

Testing:

• Added test cases to verify checkpoint behavior with manual optimization
• Ensured backward compatibility with automatic optimization
• Verified warning messages are shown in appropriate scenarios

---

📚 Documentation preview 📚: https://pytorch-lightning--21239.org.readthedocs.build/en/21239/

[littlebullGit]

The link error is (generated/CONTRIBUTING: line 6) broken https://medium.com/pytorch-lightning/quick-contribution-guide-86d977171b3a - 429 Client Error: Too Many Requests for url: https://medium.com/pytorch-lightning/quick-contribution-guide-86d977171b3a. Not related to my code. The other one is just timed out.
@Borda @SkafteNicki let me know how to proceed.

[SkafteNicki]

The link error is (generated/CONTRIBUTING: line 6) broken https://medium.com/pytorch-lightning/quick-contribution-guide-86d977171b3a - 429 Client Error: Too Many Requests for url: https://medium.com/pytorch-lightning/quick-contribution-guide-86d977171b3a. Not related to my code. The other one is just timed out.
@Borda @SkafteNicki let me know how to proceed.

Our CI is broken at the moment, nothing you can do. Please stand by while it being fixed.

[littlebullGit]

@SkafteNicki @justusschock can we try CI again ? the last run seems fine to me.