How to execute a file from another file with if __name__ == '__main__' for using multiprocessing

Question

I am currently trying to parallelize a piece of code that trains a neural network with pytorch. For that I want to use the multiprocessing library. I now want to use a code ensemble of the following structure:

test1.py file:

import runpy

runpy.run_module('test2', run_name= '__main__')

test2.py file:

import multiprocessing as mp
from test3 import function

a = 1
b = 2

print(__name__)

def main():
    print(__name__)
    if __name__ == '__main__':
        ctx = mp.get_context('spawn')
        index = [1,2,3,4]
    
        for i in index:
            processes = []
            process = ctx.Process(target=function, args=(a,b))
            process.start()
            processes.append(process)
        for process in processes:
            process.join()
        
    main()

test3.py file:

def function(a,b):
print(a+b)

For executing test2.py explicitly everything goes as expected and one gets:

__main__
__main__
__mp_main__
__mp_main__
3
__mp_main__
__mp_main__
3
__mp_main__
__mp_main__
3
__mp_main__
__mp_main__
3

Whereas for executing file test1.py the __name__ attribute does not get changed for the child interpreter opening a process and one gets an error like:

RuntimeError: 
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.

Which should actually be resolved by the if __name__ == '__main__': line, but is not in that case. My questions now are:

1. Why does that not work woth using runpy, and more importantly:
2. How can I resolve this still executing the test2.py file from another file? Is runpy the problem, and another way to execute the file better?

Sorry, if the question is pretty trivial, but I am still a beginner.

If important, I am working on ubuntu using spyder.

Oh, and if anybody asks why I use spawn on ubuntu: The pytorch pachage unfortunately only supports the spawn context.