add print_operators_doc in travis ci

Author: luotao1

paddle/scripts/travis/build_doc.sh

@@ -10,6 +10,8 @@ cmake .. -DCMAKE_BUILD_TYPE=Debug -DWITH_GPU=OFF -DWITH_MKL=OFF -DWITH_DOC=ON
 make -j `nproc` gen_proto_py
 make -j `nproc` paddle_python
 make -j `nproc` paddle_docs paddle_docs_cn
+make -j `nproc` print_operators_doc
+paddle/pybind/print_operators_doc > doc/en/html/operators.json
 
 # check websites for broken links
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