Reactivate Megatron tests an use less workers

.circleci/config.yml

@@ -145,7 +145,7 @@ jobs:
                   key: v0.4-torch-{{ checksum "setup.py" }}
                   paths:
                       - '~/.cache/pip'
-            - run: python -m pytest -n 8 --dist=loadfile -s --make-reports=tests_torch ./tests/ | tee tests_output.txt
+            - run: python -m pytest -n 4 --dist=loadfile -s --make-reports=tests_torch ./tests/ | tee tests_output.txt
             - store_artifacts:
                   path: ~/transformers/tests_output.txt
             - store_artifacts:

tests/test_modeling_megatron_bert.py

@@ -265,7 +265,6 @@ class MegatronBertModelTester:
 
 
 @require_torch
-@unittest.skip("Temporary skip to make the CI pass reliably.")
 class MegatronBertModelTest(ModelTesterMixin, unittest.TestCase):
 
     all_model_classes = (