Refactor assertions that use walrus

[awaelchli]

What does this PR do?

Fixes #18495
I couldn't find an existing rule for the linter that would prevent the use of walrus in assertions, so I asked if this could be useful in general: astral-sh/ruff#7205

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---

📚 Documentation preview 📚: https://pytorch-lightning--18496.org.readthedocs.build/en/18496/

cc @Borda @carmocca @justusschock @awaelchli

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 pytorch_lightning: Tests workflow

Check ID	Status
pl-cpu (macOS-11, lightning, 3.8, 1.11)	success	✅
pl-cpu (macOS-11, lightning, 3.9, 1.12)	success	✅
pl-cpu (macOS-11, lightning, 3.10, 1.13)	success	✅
pl-cpu (macOS-11, lightning, 3.10, 2.0)	success	✅
pl-cpu (macOS-11, lightning, 3.8, 1.11, oldest)	success	✅
pl-cpu (ubuntu-20.04, lightning, 3.8, 1.11)	success	✅
pl-cpu (ubuntu-20.04, lightning, 3.9, 1.12)	success	✅
pl-cpu (ubuntu-20.04, lightning, 3.10, 1.13)	success	✅
pl-cpu (ubuntu-20.04, lightning, 3.10, 2.0)	success	✅
pl-cpu (ubuntu-20.04, lightning, 3.8, 1.11, oldest)	success	✅
pl-cpu (windows-2022, lightning, 3.8, 1.11)	success	✅
pl-cpu (windows-2022, lightning, 3.9, 1.12)	success	✅
pl-cpu (windows-2022, lightning, 3.10, 1.13)	success	✅
pl-cpu (windows-2022, lightning, 3.10, 2.0)	success	✅
pl-cpu (windows-2022, lightning, 3.8, 1.11, oldest)	success	✅
pl-cpu (macOS-11, pytorch, 3.8, 1.13)	success	✅
pl-cpu (ubuntu-20.04, pytorch, 3.8, 1.13)	success	✅
pl-cpu (windows-2022, pytorch, 3.8, 1.13)	success	✅
pl-cpu (macOS-12, pytorch, 3.11, 2.0)	success	✅
pl-cpu (ubuntu-22.04, pytorch, 3.11, 2.0)	success	✅
pl-cpu (windows-2022, pytorch, 3.11, 2.0)	success	✅

These checks are required after the changes to src/lightning/fabric/strategies/xla_fsdp.py, src/lightning/pytorch/loops/evaluation_loop.py, src/lightning/pytorch/loops/fetchers.py, src/lightning/pytorch/loops/fit_loop.py, src/lightning/pytorch/loops/prediction_loop.py, tests/tests_pytorch/run_standalone_tasks.sh.

🟢 pytorch_lightning: Azure GPU

Check ID	Status
[pytorch-lightning (GPUs) (testing Lightning	latest)](https://dev.azure.com/Lightning-AI/72ab7ed8-b00f-4b6e-b131-3388f7ffafa7/_build/results?buildId=173529&view=logs&jobId=47e66f3c-897a-5428-da11-bf5c7745762e)	success
[pytorch-lightning (GPUs) (testing PyTorch	latest)](https://dev.azure.com/Lightning-AI/72ab7ed8-b00f-4b6e-b131-3388f7ffafa7/_build/results?buildId=173529&view=logs&jobId=3f274fac-2e11-54ca-487e-194c91f3ae9f)	success

These checks are required after the changes to src/lightning/pytorch/loops/evaluation_loop.py, src/lightning/pytorch/loops/fetchers.py, src/lightning/pytorch/loops/fit_loop.py, src/lightning/pytorch/loops/prediction_loop.py, tests/tests_pytorch/run_standalone_tasks.sh, src/lightning/fabric/strategies/xla_fsdp.py.

🟢 pytorch_lightning: Benchmarks

Check ID	Status
lightning.Benchmarks	success	✅

These checks are required after the changes to src/lightning/fabric/strategies/xla_fsdp.py, src/lightning/pytorch/loops/evaluation_loop.py, src/lightning/pytorch/loops/fetchers.py, src/lightning/pytorch/loops/fit_loop.py, src/lightning/pytorch/loops/prediction_loop.py.

🟢 fabric: Docs

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docs-make (fabric, doctest)	success	✅
docs-make (fabric, html)	success	✅

These checks are required after the changes to src/lightning/fabric/strategies/xla_fsdp.py.

🟢 pytorch_lightning: Docs

Check ID	Status
docs-make (pytorch, doctest)	success	✅
docs-make (pytorch, html)	success	✅

These checks are required after the changes to src/lightning/pytorch/loops/evaluation_loop.py, src/lightning/pytorch/loops/fetchers.py, src/lightning/pytorch/loops/fit_loop.py, src/lightning/pytorch/loops/prediction_loop.py.

🟢 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, 3.10, 2.0)	success	✅
fabric-cpu (macOS-11, lightning, 3.8, 1.11, oldest)	success	✅
fabric-cpu (ubuntu-20.04, lightning, 3.8, 1.11)	success	✅
fabric-cpu (ubuntu-20.04, lightning, 3.9, 1.12)	success	✅
fabric-cpu (ubuntu-20.04, lightning, 3.10, 1.13)	success	✅
fabric-cpu (ubuntu-20.04, lightning, 3.10, 2.0)	success	✅
fabric-cpu (ubuntu-20.04, lightning, 3.8, 1.11, oldest)	success	✅
fabric-cpu (windows-2022, lightning, 3.8, 1.11)	success	✅
fabric-cpu (windows-2022, lightning, 3.9, 1.12)	success	✅
fabric-cpu (windows-2022, lightning, 3.10, 1.13)	success	✅
fabric-cpu (windows-2022, lightning, 3.10, 2.0)	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	✅
fabric-cpu (macOS-12, fabric, 3.11, 2.0)	success	✅
fabric-cpu (ubuntu-22.04, fabric, 3.11, 2.0)	success	✅
fabric-cpu (windows-2022, fabric, 3.11, 2.0)	success	✅

These checks are required after the changes to src/lightning/fabric/strategies/xla_fsdp.py.

🟢 lightning_fabric: Azure GPU

Check ID	Status
[lightning-fabric (GPUs) (testing Fabric	latest)](https://dev.azure.com/Lightning-AI/72ab7ed8-b00f-4b6e-b131-3388f7ffafa7/_build/results?buildId=173531&view=logs&jobId=3f274fac-2e11-54ca-487e-194c91f3ae9f)	success
[lightning-fabric (GPUs) (testing Lightning	latest)](https://dev.azure.com/Lightning-AI/72ab7ed8-b00f-4b6e-b131-3388f7ffafa7/_build/results?buildId=173531&view=logs&jobId=2a86d09f-f187-5efb-70b9-b75304f1c617)	success

These checks are required after the changes to src/lightning/fabric/strategies/xla_fsdp.py.

🟢 mypy

Check ID	Status
mypy	success	✅

These checks are required after the changes to src/lightning/fabric/strategies/xla_fsdp.py, src/lightning/pytorch/loops/evaluation_loop.py, src/lightning/pytorch/loops/fetchers.py, src/lightning/pytorch/loops/fit_loop.py, src/lightning/pytorch/loops/prediction_loop.py.

🟢 install

Check ID	Status
install-pkg (ubuntu-22.04, app, 3.8)	success	✅
install-pkg (ubuntu-22.04, app, 3.11)	success	✅
install-pkg (ubuntu-22.04, fabric, 3.8)	success	✅
install-pkg (ubuntu-22.04, fabric, 3.11)	success	✅
install-pkg (ubuntu-22.04, pytorch, 3.8)	success	✅
install-pkg (ubuntu-22.04, pytorch, 3.11)	success	✅
install-pkg (ubuntu-22.04, lightning, 3.8)	success	✅
install-pkg (ubuntu-22.04, lightning, 3.11)	success	✅
install-pkg (ubuntu-22.04, notset, 3.8)	success	✅
install-pkg (ubuntu-22.04, notset, 3.11)	success	✅
install-pkg (macOS-12, app, 3.8)	success	✅
install-pkg (macOS-12, app, 3.11)	success	✅
install-pkg (macOS-12, fabric, 3.8)	success	✅
install-pkg (macOS-12, fabric, 3.11)	success	✅
install-pkg (macOS-12, pytorch, 3.8)	success	✅
install-pkg (macOS-12, pytorch, 3.11)	success	✅
install-pkg (macOS-12, lightning, 3.8)	success	✅
install-pkg (macOS-12, lightning, 3.11)	success	✅
install-pkg (macOS-12, notset, 3.8)	success	✅
install-pkg (macOS-12, notset, 3.11)	success	✅
install-pkg (windows-2022, app, 3.8)	success	✅
install-pkg (windows-2022, app, 3.11)	success	✅
install-pkg (windows-2022, fabric, 3.8)	success	✅
install-pkg (windows-2022, fabric, 3.11)	success	✅
install-pkg (windows-2022, pytorch, 3.8)	success	✅
install-pkg (windows-2022, pytorch, 3.11)	success	✅
install-pkg (windows-2022, lightning, 3.8)	success	✅
install-pkg (windows-2022, lightning, 3.11)	success	✅
install-pkg (windows-2022, notset, 3.8)	success	✅
install-pkg (windows-2022, notset, 3.11)	success	✅

These checks are required after the changes to src/lightning/fabric/strategies/xla_fsdp.py, src/lightning/pytorch/loops/evaluation_loop.py, src/lightning/pytorch/loops/fetchers.py, src/lightning/pytorch/loops/fit_loop.py, src/lightning/pytorch/loops/prediction_loop.py.

---

Thank you for your contribution! 💜

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[codecov]

Codecov Report

Merging #18496 (6b71bbc) into master (cf437ed) will decrease coverage by 30%.
Report is 1 commits behind head on master.
The diff coverage is 88%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #18496      +/-   ##
==========================================
- Coverage      85%      54%     -30%     
==========================================
  Files         427      422       -5     
  Lines       33337    33252      -85     
==========================================
- Hits        28200    18089   -10111     
- Misses       5137    15163   +10026