update relay python api doc

[yongwww]

Fix for issue #2644

[yongwww]

@tqchen @zhiics @wweic @yzhliu pls help take a look. Not sure if the code snippet below works (if not, pls suggest... sorry I am not familiar with Sphinx), trying to link to relay op list doc, it makes no sense to maintain two copies

  .. toctree::
  :maxdepth: 2

  langref/relay_op

[tqchen]

We do need t make keep relay/nn.rst and relay/vision.rst

[tqchen]

For now, let us not cross ref, let us keep the relay.nn and relay.vision namespace because they are what are used directly by the user. Let us not refer to relay_op for now, since that will result in a too deep nesting.

You can build the doc locally to verify

We can bring other changes.

[wweic]

@yongwww after you make the change, try following command to build docs locally(from Jenkins script). Then verify the docs in browser.

mkdir -p build
cd build
cp ../cmake/config.cmake .
echo 'set(USE_SORT' 'ON)' >> config.cmake
echo 'set(USE_GRAPH_RUNTIME_DEBUG' 'ON)' >> config.cmake
echo 'set(USE_LLVM' 'llvm-config-4.0)' >> config.cmake
echo 'set(USE_NNPACK' 'ON)' >> config.cmake
echo 'set(NNPACK_PATH' '/NNPACK/build/)' >> config.cmake
echo 'set(USE_ANTLR' 'ON)' >> config.cmake
echo 'set(CMAKE_CXX_COMPILER' 'g++)' >> config.cmake
echo 'set(CMAKE_CXX_FLAGS' '-Werror)' >> config.cmake
cd ..
docker/bash.sh tvmai/ci-cpu ./tests/scripts/task_build.sh build -j2
docker/bash.sh tvmai/ci-cpu ./tests/scripts/task_python_docs.sh