[App] Enable properties for the Lightning flow

[tchaton]

What does this PR do?

The LightningFlow overrides the setattr method and didn't call property.fset properly. The test below breaks on master.

class FlowValue(LightningFlow):
    def __init__(self):
        super().__init__()
        self._value = None
        self._has_found = False

    @property
    def value(self):
        return self._value

    @value.setter
    def value(self, value):
        self._value = value

    def run(self):
        if self.value is None:
            self.value = True

    def __setattr__(self, name, value):
        if name == "_value" and value is True:
            self._has_found = True
        super().__setattr__(name, value)


def test_lightning_flow_properties():
    """Validates setting properties to the LightningFlow properly calls property.fset."""

    flow = FlowValue()
    assert not flow._has_found
    flow.run()
    assert flow._has_found

tests/tests_app/core/test_lightning_app.py:1143: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
tests/tests_app/core/test_lightning_app.py:1130: in run
    self.value = True
tests/tests_app/core/test_lightning_app.py:1135: in __setattr__
    super().__setattr__(name, value)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <tests_app.core.test_lightning_app.FlowValue object at 0x134920e50>, name = 'value', value = True

    def __setattr__(self, name, value):
        from lightning_app.structures import Dict, List
    
        if (
            not _is_init_context(self)
            and name not in self._state
            and name not in self._paths
            and (
                not isinstance(value, (LightningWork, LightningFlow))
                or (isinstance(value, (LightningWork, LightningFlow)) and not _is_run_context(self))
            )
            and name not in self._works.union(self._flows)
            and self._is_state_attribute(name)
        ):
>           raise AttributeError(f"Cannot set attributes that were not defined in __init__: {name}")
E           AttributeError: Cannot set attributes that were not defined in __init__: value

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• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

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Did you have fun?

Make sure you had fun coding 🙃

cc @Borda

[ethanwharris]

Looks good, not sure it needs two new private methods

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 lightning_app: Tests workflow

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

🟢 lightning_app: Examples

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

🟢 lightning_app: Azure

Check ID	Status
App.cloud-e2e	success	✅

🟢 lightning_app: Docs

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

🟢 mypy

Check ID	Status
mypy	success	✅

🟢 install

Check ID	Status
install-pkg (ubuntu-22.04, app, 3.7)	success	✅
install-pkg (ubuntu-22.04, app, 3.10)	success	✅
install-pkg (ubuntu-22.04, lite, 3.7)	success	✅
install-pkg (ubuntu-22.04, lite, 3.10)	success	✅
install-pkg (ubuntu-22.04, pytorch, 3.7)	success	✅
install-pkg (ubuntu-22.04, pytorch, 3.10)	success	✅
install-pkg (ubuntu-22.04, lightning, 3.7)	success	✅
install-pkg (ubuntu-22.04, lightning, 3.10)	success	✅
install-pkg (macOS-12, app, 3.7)	success	✅
install-pkg (macOS-12, app, 3.10)	success	✅
install-pkg (macOS-12, lite, 3.7)	success	✅
install-pkg (macOS-12, lite, 3.10)	success	✅
install-pkg (macOS-12, pytorch, 3.7)	success	✅
install-pkg (macOS-12, pytorch, 3.10)	success	✅
install-pkg (macOS-12, lightning, 3.7)	success	✅
install-pkg (macOS-12, lightning, 3.10)	success	✅
install-pkg (windows-2022, app, 3.7)	success	✅
install-pkg (windows-2022, app, 3.10)	success	✅
install-pkg (windows-2022, lite, 3.7)	success	✅
install-pkg (windows-2022, lite, 3.10)	success	✅
install-pkg (windows-2022, pytorch, 3.7)	success	✅
install-pkg (windows-2022, pytorch, 3.10)	success	✅
install-pkg (windows-2022, lightning, 3.7)	success	✅
install-pkg (windows-2022, lightning, 3.10)	success	✅

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