modal.functions.gather type-aware via generics

[zsiegel92]

Now that .spawn() is type-aware via this PR, outputs = modal.functions.gather(*[my_function.spawn(job) for job in jobs]) is accepted by type-checkers, and since .spawn returns a generic FunctionCall[R] (where my_function returns R), it should be possible for outputs to be type-checked to the return type of my_function without any additional annotations.

Would have to make the gather function generic.

Note - I manually edit the generated functions.pyi in the Modal package to have this:

class __gather_spec(typing_extensions.Protocol):
    def __call__(self, *function_calls: FunctionCall[R])->typing.Iterable[R]: ...

making my IDE the only IDE that knows the type of gather(*fn_calls), which is nice (the annotation may be a bit wrong idk).

I'm not sure how y'all generate your .pyi files but maybe you could have a build step that edits them to match the true behavior; the modal SDK annotations seem to be overly pessimistic/broad.

[freider]

Yeah I think that would be a nice improvement and I think it should work, I can make a quick patch to see!

FYI the .pyi files are generated by calling inv type-stubs in the client repo (requires requirements.dev.txt to be installed).

[zsiegel92]

Thanks @freider for moving this along! Big quality of life improvement for me.