[ci] Simplify and annotate with comments

.gitlab-ci.yml

@@ -13,12 +13,14 @@ variables:
 
 documentation:
   before_script:
+    # for opencv-python dependence
     - apt-get update && apt-get install -y libgl1-mesa-glx > /dev/null
-    - export OMP_NUM_THREADS=1
     - !reference [.snippets, doc-prepare]
 
 tests:
   before_script:
+    # for opencv-python dependence
     - if [ "$TAG" == "docker" ]; then apt-get update && apt-get install -y libgl1-mesa-glx > /dev/null; fi
+    # for pytorch quirks on linux
     - if [ "$TAG" == "docker" ]; then export OMP_NUM_THREADS=1; fi # Set the number of threads used to 1
     - !reference [.snippets, test-prepare]