[App] Enable debugger with LightningApp

[tchaton]

What does this PR do?

This PR enables running an app within a debugger with breakpoints with flows and works without your changing code :)

fimal.mp4

Does your PR introduce any breaking changes? If yes, please list them.

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Did you have fun?

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cc @Borda

[ethanwharris]

LGTM 😃

[lantiga]

Great! Docstring change and a comment

[awaelchli]

I don't understand how this is supposed to work. I checked out the branch, set a breakpoint() in the examples/app_boring in one of the works, and it exited with an error.

Perhaps you only meant to add support for the debugger in the IDE. I verified that it works in PyCharm with the debugger, but the user will have to change their code from

    app = L.LightningApp(BoringApp())

to

    app = L.LightningApp(BoringApp())
    MultiProcessRuntime(app).dispatch()

in order to run it as a regular python program without cli.
However, we never explain to the user that this is another way of launching an app locally. Using the runtime directly like that is only meant for our testing. We would need to introduce a simpler API here to do the same if we really want to support running it like that.

[tchaton]

I don't understand how this is supposed to work. I checked out the branch, set a breakpoint() in the examples/app_boring in one of the works, and it exited with an error.

Perhaps you only meant to add support for the debugger in the IDE. I verified that it works in PyCharm with the debugger, but the user will have to change their code from

    app = L.LightningApp(BoringApp())

to

    app = L.LightningApp(BoringApp())
    MultiProcessRuntime(app).dispatch()

in order to run it as a regular python program without cli. However, we never explain to the user that this is another way of launching an app locally. Using the runtime directly like that is only meant for our testing. We would need to introduce a simpler API here to do the same if we really want to support running it like that.

This is meant to work if you run your script with a debugger attached to it. This enables to debug very simply your app directly within the IDE. You don't need to change single line of code for it to work, this is the entire goal here.

in order to run it as a regular python program without cli.

This isn't what we are trying to enable. Just for users to run their app in their IDE without code modification and set breakpoints wherever they decide.