Apply import formatter `isort`
🚀 Refactoring
As isort has been added to ci in #4242, we now need to apply the formatter step by step i.e. a submodule per PR (recommended in #4242 (comment) by @Borda)

Steps
For each PR:

choose one submodule from below list and apply isort to it
remove the corresponding line in pyproject.toml
make sure isort --check passes



Progress
The rest of the submodules should be done by @arnaudgelas.
The followings are PL submodules.

 pytorch_lightning/*.py >> #4717
 pytorch_lightning/accelerators/
 pytorch_lightning/callbacks/
 pytorch_lightning/cluster_environments/
 pytorch_lightning/core/
 pytorch_lightning/distributed/
 pytorch_lightning/loggers/
 pytorch_lightning/metrics/
 pytorch_lightning/overrides/
 pytorch_lightning/plugins/
 pytorch_lightning/profiler/
 pytorch_lightning/trainer/
 pytorch_lightning/tuner/
 pytorch_lightning/utilities/

The followings are tests.

 tests/*.py >> #4717
 tests/backends/ >> #5430
 tests/base/ >> #5429
 tests/callbacks/ >> #5428
 tests/checkpointing/ >> #5427
 tests/core/ >> #5426
 tests/loggers/ >> #5425
 tests/plugins/ >> #5422
 tests/metrics/ >> #5424
 tests/models/ >> #5423
 tests/trainer/ >> #5421
 tests/tuner/
 tests/utilities/ >> #5420

The followings are other python files.

 *.py (#4242)
 docs/ (#4242)
 pl_examples/ (#5291)