Bring back `composite` documentation (#331)

* update

* update

docs/source/conf.py

 import datetime
-import sphinx_rtd_theme
 import doctest
+
+import sphinx_rtd_theme
 import torch_scatter
 
 extensions = [
     'sphinx.ext.autodoc',
-    'sphinx.ext.doctest',
     'sphinx.ext.intersphinx',
     'sphinx.ext.mathjax',
     'sphinx.ext.napoleon',
     'sphinx.ext.viewcode',
-    'sphinx.ext.githubpages',
 ]
 
 source_suffix = '.rst'

docs/source/functions/composite.rst

+composite
+=========
+
+.. currentmodule:: torch_scatter.composite
+
+.. automodule:: torch_scatter.composite
+   :members:
+   :undoc-members:

docs/source/index.rst

@@ -14,7 +14,10 @@ All included operations are broadcastable, work on varying data types, are imple
    :maxdepth: 0
    :caption: Package reference
 
-   functions/*
+   functions/scatter
+   functions/segment_coo
+   functions/segment_csr
+   functions/composite
 
 Indices and tables
 ==================