Synchronize gradients in manual optimization with DDPStrategy(static_graph=True)

[Sohaib-Ahmed21]

What does this PR do?

Fixes gradient synchronization when using manual optimization with DDPStrategy(static_graph=True).

• Calls reducer._delay_all_reduce() in post_backward to ensure gradients are properly reduced in the first iteration.
• Adds a regression test (test_ddp_gradients_synced) that checks gradient synchronization across:
  • automatic_optimization=True / False
  • static_graph=True / False

Fixes #18086

Before submitting

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  Yes
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• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you write any new necessary tests? (not for typos and docs)
  yes
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• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

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📚 Documentation preview 📚: https://pytorch-lightning--21251.org.readthedocs.build/en/21251/

[Sohaib-Ahmed21]

Failures seem unrelated to the PR, can you please review the PR @Borda @SkafteNicki , thanks!

[SkafteNicki]

Failures seem unrelated to the PR, can you please review the PR @Borda @SkafteNicki , thanks!

@Sohaib-Ahmed21 yes CI is down at the moment so nothing to do at the moment

[Sohaib-Ahmed21]

@Borda @SkafteNicki all tests are passing now. Can you please review this PR, thanks!

[Sohaib-Ahmed21]

@SkafteNicki @Borda @lantiga , gentle ping on this PR. I believe it's ready for review when you have a moment. Thanks!

[Borda]

@SkafteNicki @Borda @lantiga , gentle ping on this PR. I believe it's ready for review when you have a moment. Thanks!

Apology, I am not a code-owner anymore...

[SkafteNicki]

LGTM :)