Update documentation and fix docstrings (#788)

[vincentqb]

• Addresses Backend functions not in documentation #549 ComputeDeltas equation is not rendered #638 Issues with documentation generations #786
• Add torchaudio top level module doc
• Separate torchaudio top level module doc from index.html
• Add backend module doc.
• Remove -> None from function signature as it adds noise to documentation
• Changed function argument name of torchaudio.backend.sox_io_backend.save from tensor to src, so that it matches with the reset of backends.
• Tweak bunch of docstrings

[mthrok]

Looks good.

[vincentqb]

For reference, how to build docs for website:

conda create -n torchaudiodocs
conda activate torchaudiodocs

conda install sphinx-build
conda install sphinx
conda install -c pytorch pytorch
pip install sphinxcontrib-katex

# Install latest pytorch_sphinx_theme in conda environment

# Get a fresh copy of torchaudio
git clone https://github.com/pytorch/audio audio-docs
git checkout origin/release/0.6
cd audio-docs/docs
make html
git checkout origin/gh-pages
cd ..
cp -rp docs/build/html/* ./
git add .
git commit -m"update docs for 0.6.0"
git push origin gh-pages