🐛 fix bug

## Building the Documentation

To build the docs, you can use the `Makefile` that's been added to

this directory. (Note that `deepchem` must be installed first.) To

generate docs in html, run

this directory. To generate docs in html, run following commands.

```

$ pip install -r ../requirements-docs.txt

$ open build/html/index.html

```

If you want to confirm logs in more detail

If you want to confirm logs in more details

```

$ make clean html SPHINXOPTS=-vvv

    'sphinx.ext.linkcode', 'sphinx.ext.mathjax',

]

# Options for autodoc directives

autodoc_default_options = {

    'member-order': 'bysource',

    'special-members': True,

    'exclude-members': '__repr__, __str__, __weakref__',

}

autodoc_typehints = "description"

# How to represents typehints.

autodoc_typehints = "signature"

mathjax_path = 'http://mathjax.connectmv.com/MathJax.js?config=default'

# The theme to use for HTML and HTML Help pages.  See the documentation for

# a list of builtin themes.

html_theme = 'sphinx_rtd_theme'

html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]

# of the sidebar.

html_logo = '_static/logo.png'

# Customize the sphinx theme

html_theme_options = {

    'collapse_navigation': False,

    'display_version': True,