fix problem of memory issue (#487)

* fix problem of memory issue

* remove check

* Sequential the run

* change n_job

Author: lionelkusch

docs/Makefile

@@ -2,7 +2,8 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    = -j auto -W --keep-going -n 
+N_JOB         = auto
+SPHINXOPTS    = -j ${N_JOB} -W --keep-going -n 
 # -j/--jobs:		    N processes in parallel (auto=number of CPUs)
 # -W/--fail-on-warning: Turn warnings into errors.
 # --keep-going:         Runs sphinx-build to completion and exits  

tools/documentation/circleci/build_doc.sh

@@ -114,7 +114,7 @@ else
 fi
 
 # The pipefail is requested to propagate exit code
-set -o pipefail && cd docs && make $make_args 2>&1 | tee ~/output_sphinx.txt
+set -o pipefail && cd docs && make N_JOB=2 $make_args 2>&1 | tee ~/output_sphinx.txt
 cd -
 
 set +o pipefail