[App] Refactor plugins to be a standalone LightningPlugin

[ethanwharris]

What does this PR do?

Changes plugins to be standalone, similar to apps - rather than children of apps.

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cc @Borda @justusschock @awaelchli

[tchaton]

LGTM !

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 lightning_app: Tests workflow

Check ID	Status
app-pytest (macOS-11, lightning, 3.8, latest)	success	✅
app-pytest (macOS-11, lightning, 3.8, oldest)	success	✅
app-pytest (macOS-11, app, 3.9, latest)	success	✅
app-pytest (ubuntu-20.04, lightning, 3.8, latest)	success	✅
app-pytest (ubuntu-20.04, lightning, 3.8, oldest)	success	✅
app-pytest (ubuntu-20.04, app, 3.9, latest)	success	✅
app-pytest (windows-2022, lightning, 3.8, latest)	success	✅
app-pytest (windows-2022, lightning, 3.8, oldest)	success	✅
app-pytest (windows-2022, app, 3.8, latest)	success	✅

These checks are required after the changes to src/lightning/app/__init__.py, src/lightning/app/core/__init__.py, src/lightning/app/core/flow.py, src/lightning/app/core/plugin.py, src/lightning/app/runners/cloud.py, src/lightning/app/runners/multiprocess.py, src/lightning/app/utilities/commands/base.py, src/lightning/app/utilities/load_app.py, tests/tests_app/cli/test_run_app.py, tests/tests_app/core/lightning_app/test_configure_layout.py, tests/tests_app/core/test_plugin.py, tests/tests_app/runners/test_cloud.py.

🟢 lightning_app: Examples

Check ID	Status
app-examples (macOS-11, lightning, 3.9, latest)	success	✅
app-examples (macOS-11, lightning, 3.9, oldest)	success	✅
app-examples (macOS-11, app, 3.9, latest)	success	✅
app-examples (ubuntu-20.04, lightning, 3.9, latest)	success	✅
app-examples (ubuntu-20.04, lightning, 3.9, oldest)	success	✅
app-examples (ubuntu-20.04, app, 3.9, latest)	success	✅
app-examples (windows-2022, lightning, 3.9, latest)	success	✅
app-examples (windows-2022, lightning, 3.9, oldest)	success	✅
app-examples (windows-2022, app, 3.9, latest)	success	✅

These checks are required after the changes to src/lightning/app/__init__.py, src/lightning/app/core/__init__.py, src/lightning/app/core/flow.py, src/lightning/app/core/plugin.py, src/lightning/app/runners/cloud.py, src/lightning/app/runners/multiprocess.py, src/lightning/app/utilities/commands/base.py, src/lightning/app/utilities/load_app.py.

🟢 lightning_app: Azure

Check ID	Status
App.cloud-e2e	success	✅

These checks are required after the changes to src/lightning/app/__init__.py, src/lightning/app/core/__init__.py, src/lightning/app/core/flow.py, src/lightning/app/core/plugin.py, src/lightning/app/runners/cloud.py, src/lightning/app/runners/multiprocess.py, src/lightning/app/utilities/commands/base.py, src/lightning/app/utilities/load_app.py.

🟢 lightning_app: Docs

Check ID	Status
make-doctest (lightning)	success	✅
make-html (lightning)	success	✅

These checks are required after the changes to src/lightning/app/__init__.py, src/lightning/app/core/__init__.py, src/lightning/app/core/flow.py, src/lightning/app/core/plugin.py, src/lightning/app/runners/cloud.py, src/lightning/app/runners/multiprocess.py, src/lightning/app/utilities/commands/base.py, src/lightning/app/utilities/load_app.py.

🟢 mypy

Check ID	Status
mypy	success	✅

These checks are required after the changes to src/lightning/app/__init__.py, src/lightning/app/core/__init__.py, src/lightning/app/core/flow.py, src/lightning/app/core/plugin.py, src/lightning/app/runners/cloud.py, src/lightning/app/runners/multiprocess.py, src/lightning/app/utilities/commands/base.py, src/lightning/app/utilities/load_app.py.

🟢 install

Check ID	Status
install-pkg (ubuntu-22.04, app, 3.8)	success	✅
install-pkg (ubuntu-22.04, app, 3.10)	success	✅
install-pkg (ubuntu-22.04, fabric, 3.8)	success	✅
install-pkg (ubuntu-22.04, fabric, 3.10)	success	✅
install-pkg (ubuntu-22.04, pytorch, 3.8)	success	✅
install-pkg (ubuntu-22.04, pytorch, 3.10)	success	✅
install-pkg (ubuntu-22.04, lightning, 3.8)	success	✅
install-pkg (ubuntu-22.04, lightning, 3.10)	success	✅
install-pkg (ubuntu-22.04, notset, 3.8)	success	✅
install-pkg (ubuntu-22.04, notset, 3.10)	success	✅
install-pkg (macOS-12, app, 3.8)	success	✅
install-pkg (macOS-12, app, 3.10)	success	✅
install-pkg (macOS-12, fabric, 3.8)	success	✅
install-pkg (macOS-12, fabric, 3.10)	success	✅
install-pkg (macOS-12, pytorch, 3.8)	success	✅
install-pkg (macOS-12, pytorch, 3.10)	success	✅
install-pkg (macOS-12, lightning, 3.8)	success	✅
install-pkg (macOS-12, lightning, 3.10)	success	✅
install-pkg (macOS-12, notset, 3.8)	success	✅
install-pkg (macOS-12, notset, 3.10)	success	✅
install-pkg (windows-2022, app, 3.8)	success	✅
install-pkg (windows-2022, app, 3.10)	success	✅
install-pkg (windows-2022, fabric, 3.8)	success	✅
install-pkg (windows-2022, fabric, 3.10)	success	✅
install-pkg (windows-2022, pytorch, 3.8)	success	✅
install-pkg (windows-2022, pytorch, 3.10)	success	✅
install-pkg (windows-2022, lightning, 3.8)	success	✅
install-pkg (windows-2022, lightning, 3.10)	success	✅
install-pkg (windows-2022, notset, 3.8)	success	✅
install-pkg (windows-2022, notset, 3.10)	success	✅

These checks are required after the changes to src/lightning/app/__init__.py, src/lightning/app/core/__init__.py, src/lightning/app/core/flow.py, src/lightning/app/core/plugin.py, src/lightning/app/runners/cloud.py, src/lightning/app/runners/multiprocess.py, src/lightning/app/utilities/commands/base.py, src/lightning/app/utilities/load_app.py.

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[awaelchli]

@ethanwharris Since this will probably merge soon, could I ask you to then cherry-pick that commit over into this branch: #16790 for an efficient releasing in the coming days. Thanks in advance!