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 pytorch#7318

- Add manual tensor() annotation, thank you, @elliotwite.
- Create class stubs for device etc.
- Improve type mapping. Thank you, Simon, for the review comments.

- Add test_type_hints.py: For testing we compile the docstring
  examples into one large file and run mypy over it.
- Make the test pass by adding lots of cases to gen_pyi.py.

- Previously, type hints didn't work for 'develop' builds,
  because the pyi was put into a generated directory and
  consequently not picked up by the Python interpreter.
  It's a lot easier, and consistent with how we generate
  other files in PyTorch, to just generate the file in place.
  I need to test if the pyi file actually gets installed.

- I figured out why mypy didn't work in the default directory;
  it's because we had a mypy.ini which specified Python 2.
  I fixed this by making our test file Python 2 syntax compatible.

- I added some more comments, and adjusted formatting to my
  taste.

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

This is done by deleting all of annotation based inference; instead
we'll just write those signatures out manually.

A bit of code reorging happened along the way.

I still need to add back the direct Python implementations;
incoming soon!

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Signed-off-by: Edward Z. Yang <ezyang@fb.com>