Have you checked what is _VF?
Very strange:
import torch
import sys
import types


class VFModule(types.ModuleType):
    def __init__(self, name):
        super(VFModule, self).__init__(name)
        self.vf = torch._C._VariableFunctions

    def __getattr__(self, attr):
        return getattr(self.vf, attr)

sys.modules[__name__] = VFModule(__name__)

My guess is _VF will navigate you to a specified module.
Also notice there is a module called dropout.cpp:https://github.com/pytorch/pytorch/blob/d1623f4cc9334ade7376b3685bb91d3071e3b418/torch/csrc/api/src/nn/modules/dropout.cpp 279
And most of PyTorch’s computation internals are written in C++.