chore(setup): rewrite packaging

[XuehaiPan]

Description

• Rewrite packaging

  1. Totally refactor setup.py and remove function download_shared_lib. We don't need to download the .so files from an arbitrary server because they are included in the wheels (.whl).
  2. Remove git submodule pybind11 and use the PyPI package pybind11 instead. (Required by sdist).

  Now we can build from source by (requires nvcc):

  git clone https://github.com/metaopt/torchopt.git
  cd torchopt
  pip3 install .

• Update GitHub Action workflows.

  1. Install CUDA Toolkit in GitHub Action workflows. (Required by pip3 install -e .)
  2. Build wheels for Python 3.7, 3.8, 3.9, and 3.10.

• Misc: reformating files, docs update, and fixing requirements.

[Benjamin-eecs]

If you got time, maybe add the docstyle? (docsting is a mess right now I think)

[XuehaiPan]

We may open a separate PR for this.