Fix amp ddp test in Fabric

[awaelchli]

What does this PR do?

Fixes #16852

One can either mark the test standalone, or fix the strategy to ddp_spawn. I chose the latter.

cc @carmocca @justusschock @awaelchli @Borda

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 lightning_fabric: CPU workflow

Check ID	Status
fabric-cpu (macOS-11, lightning, 3.8, 1.11)	success	✅
fabric-cpu (macOS-11, lightning, 3.9, 1.12)	success	✅
fabric-cpu (macOS-11, lightning, 3.10, 1.13)	success	✅
fabric-cpu (macOS-11, lightning, 3.8, 1.11, oldest)	success	✅
fabric-cpu (ubuntu-20.04, lightning, 3.9, 1.11)	success	✅
fabric-cpu (ubuntu-20.04, lightning, 3.10, 1.12)	success	✅
fabric-cpu (ubuntu-20.04, lightning, 3.10, 1.13)	success	✅
fabric-cpu (ubuntu-20.04, lightning, 3.8, 1.11, oldest)	success	✅
fabric-cpu (windows-2022, lightning, 3.9, 1.11)	success	✅
fabric-cpu (windows-2022, lightning, 3.10, 1.12)	success	✅
fabric-cpu (windows-2022, lightning, 3.10, 1.13)	success	✅
fabric-cpu (windows-2022, lightning, 3.8, 1.11, oldest)	success	✅
fabric-cpu (macOS-11, fabric, 3.8, 1.13)	success	✅
fabric-cpu (ubuntu-20.04, fabric, 3.8, 1.13)	success	✅
fabric-cpu (windows-2022, fabric, 3.8, 1.13)	success	✅

These checks are required after the changes to tests/tests_fabric/plugins/precision/test_amp_integration.py.

🟢 lightning_fabric: Azure GPU

Check ID	Status
lightning-fabric (GPUs)	success	✅

These checks are required after the changes to tests/tests_fabric/plugins/precision/test_amp_integration.py.

---

Thank you for your contribution! 💜

Note
This comment is automatically generated and updates for 60 minutes every 180 seconds. If you have any other questions, contact carmocca for help.