Parallelize tests on Circle CI.

Set the number of CPUs manually based on the Circle CI resource class,
or else we're getting 36 CPUs, which is far too much (perhaps that's
the underlying hardware and not what Circle CI allocates to us).

Don't parallelize the custom tokenizers tests because they take less
than one second to run and parallelization actually makes them slower.

.circleci/config.yml

@@ -11,9 +11,9 @@ jobs:
             - run: sudo pip install torch
             - run: sudo pip install tensorflow
             - run: sudo pip install --progress-bar off .
-            - run: sudo pip install pytest codecov pytest-cov
+            - run: sudo pip install pytest codecov pytest-cov pytest-xdist
             - run: sudo pip install tensorboardX scikit-learn
-            - run: python -m pytest -sv ./transformers/tests/ --cov
+            - run: python -m pytest -n 8 -s -v ./transformers/tests/ --cov
             - run: codecov
     build_py3_torch:
         working_directory: ~/transformers
@@ -25,10 +25,10 @@ jobs:
             - checkout
             - run: sudo pip install torch
             - run: sudo pip install --progress-bar off .
-            - run: sudo pip install pytest codecov pytest-cov
+            - run: sudo pip install pytest codecov pytest-cov pytest-xdist
             - run: sudo pip install tensorboardX scikit-learn
-            - run: python -m pytest -sv ./transformers/tests/ --cov
-            - run: python -m pytest -sv ./examples/
+            - run: python -m pytest -n 8 -s -v ./transformers/tests/ --cov
+            - run: python -m pytest -n 8 -s -v ./examples/
             - run: codecov
     build_py3_tf:
         working_directory: ~/transformers
@@ -40,9 +40,9 @@ jobs:
             - checkout
             - run: sudo pip install tensorflow
             - run: sudo pip install --progress-bar off .
-            - run: sudo pip install pytest codecov pytest-cov
+            - run: sudo pip install pytest codecov pytest-cov pytest-xdist
             - run: sudo pip install tensorboardX scikit-learn
-            - run: python -m pytest -sv ./transformers/tests/ --cov
+            - run: python -m pytest -n 8 -s -v ./transformers/tests/ --cov
             - run: codecov
     build_py3_custom_tokenizers:
         working_directory: ~/transformers
@@ -51,7 +51,7 @@ jobs:
         steps:
             - checkout
             - run: sudo pip install --progress-bar off .
-            - run: sudo pip install pytest
+            - run: sudo pip install pytest pytest-xdist
             - run: sudo pip install mecab-python3
             - run: RUN_CUSTOM_TOKENIZERS=1 python -m pytest -sv ./transformers/tests/tokenization_bert_japanese_test.py
     deploy_doc: