* make audio requirements optional
 * add changelog for version v0.2
 * add numpy constraint for compatibility with tensorflow1.15
 * update faq
 * fix nlp requiring tensorflow
 * add torchvision to multimodal dependency
 * bump version from 0.2.1 to 0.2.2
 * add warning msg when tensorflow is not installed
 
Link: https://code.alibaba-inc.com/Ali-MaaS/MaaS-lib/codereview/9268278

1. add build_doc linter script
2. add sphinx-docs support
3. add development doc and api doc
4. change version to 0.1.0 for the first internal release version

Link: https://code.aone.alibaba-inc.com/Ali-MaaS/MaaS-lib/codereview/8775307