Add missing index.rst (#650)

docs/zh_cn/api_cn/dygraph/nas/index.rst

+.. PaddleSlim documentation master file, created by
+   sphinx-quickstart on Wed Feb  5 14:04:52 2020.
+   You can adapt this file completely to your liking, but it should at least
+   contain the root `toctree` directive.
+
+NAS
+=======
+
+.. toctree::
+   :maxdepth: 1
+
+   darts.rst

docs/zh_cn/api_cn/dygraph/ofa/index.rst

+
+Once-For-All
+=============
+
+.. toctree::
+   :maxdepth: 1
+   
+   convert_supernet_api.rst
+   ofa_api.rst
+   ofa_layer_api.rst

docs/zh_cn/api_cn/dygraph/pruners/index.rst

+Pruners
+===============
+
+.. toctree::
+   :maxdepth: 1
+
+   l1norm_filter_pruner.rst
+   l2norm_filter_pruner.rst
+   fpgm_filter_pruner.rst

docs/zh_cn/api_cn/dygraph/quanter/index.rst

+QAT
+===============
+
+.. toctree::
+   :maxdepth: 1
+
+   qat.rst