Such a conversion does not exist.
An nn.Module is a high level structure to make it convenient to write Neural networks and different operations.
A Function is an elementary element from the autograd engine that is created when doing computations on Variable and that allows to compute the gradients.
There is no one to one matching between them.
Moreover, the Function objects comes from the C++ engine, so even though you can see them from python, you should not use them to store some data as the python object is destructed as soon as it is not needed in python anymore.