Lazily import dependencies for MLFlowLogger

[awaelchli]

What does this PR do?

Refactors the mlflow logger module to lazily import its dependencies only when the MLFlowLogger instance is requested.
Part of #12786

The other logger modules will get the same treatment.

cc @justusschock @awaelchli @Borda

[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/pytorch/loggers/__init__.py, src/lightning/pytorch/loggers/mlflow.py, tests/tests_pytorch/loggers/conftest.py, tests/tests_pytorch/loggers/test_all.py, tests/tests_pytorch/loggers/test_mlflow.py.

🟢 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=174180&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=174180&view=logs&jobId=fe777007-6e77-5e50-6e71-fc5977ab193a)	success

These checks are required after the changes to src/lightning/pytorch/loggers/__init__.py, src/lightning/pytorch/loggers/mlflow.py, tests/tests_pytorch/loggers/conftest.py, tests/tests_pytorch/loggers/test_all.py, tests/tests_pytorch/loggers/test_mlflow.py.

🟢 pytorch_lightning: Benchmarks

Check ID	Status
lightning.Benchmarks	success	✅

These checks are required after the changes to src/lightning/pytorch/loggers/__init__.py, src/lightning/pytorch/loggers/mlflow.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/loggers/__init__.py, src/lightning/pytorch/loggers/mlflow.py.

🟢 mypy

Check ID	Status
mypy	success	✅

These checks are required after the changes to src/lightning/pytorch/loggers/__init__.py, src/lightning/pytorch/loggers/mlflow.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/pytorch/loggers/__init__.py, src/lightning/pytorch/loggers/mlflow.py.

---

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[codecov]

Codecov Report

Merging #18528 (d0e225d) into master (4dfc09c) will decrease coverage by 35%.
Report is 11 commits behind head on master.
The diff coverage is 56%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #18528      +/-   ##
==========================================
- Coverage      85%      50%     -35%     
==========================================
  Files         427      419       -8     
  Lines       33372    33237     -135     
==========================================
- Hits        28373    16691   -11682     
- Misses       4999    16546   +11547