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

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

[Sohaib-Ahmed21]

@SkafteNicki @Borda @lantiga kindly review this pull request, thanks!