create type hint stub files for module torch

[ezyang]

This was backported from #12500 but with tests removed and
the stub file checked in directly, so we don't have to also
make sure all build shenanigans work. Master will be properly
tested. The stub file was generated by running
'python setup.py create_pyi' and then copying the generated
pyi file in the build directory (find -name *.pyi) to torch/

Original PR description:

This is more a basis for discussion that a ready solution,
as it does lots of funny things, and for many of them
a better solution will be found.

• Initial stab at creating a type torch/init.pyi .
• We only do this for Python 3 because we want to
  use type hint introspection.
• So far, we only aim at doing this for torch functions
  and torch.Tensor.
• We need to import the newly build torch. Thus we
  do this at the end of the build.
  We use os.fork to only import the module in a child
  process.
• We use an annotate decorator to be able to put
  type hints on the native python functions in a way that
  a) they're available in the usual place for Python 3
  b) we stay Python 2 compatible
• Some annotatons in torch/functional.py are provided
  as examples, but the remaining ones still need to be done.

This could end up fixing #7318

cc @t-vi @soumith

[soumith]

@ezyang can you rebase so that tests can run

[ezyang]

Done.

[ezyang]

This is ready