maintain docs

1. build docs

   # in root directory:
   make docs
   

2. doc string format

   We adopt the google style docstring format as the standard, please refer to the following documents.

   1. Google Python style guide docstring link
   2. Google docstring example link
   3. sample：torch.nn.modules.conv link
   4. load function as an example：

   def load(file, file_format=None, **kwargs):
       """Load data from json/yaml/pickle files.
   
       This method provides a unified api for loading data from serialized files.
   
       Args:
           file (str or :obj:`Path` or file-like object): Filename or a file-like
               object.
           file_format (str, optional): If not specified, the file format will be
               inferred from the file extension, otherwise use the specified one.
               Currently supported formats include "json", "yaml/yml".
   
       Examples:
           >>> load('/path/of/your/file')  # file is stored in disk
           >>> load('https://path/of/your/file')  # file is stored on internet
           >>> load('oss://path/of/your/file')  # file is stored in petrel
   
       Returns:
           The content from the file.
       """