pin transformers version for unit tests (#2402)

Author: mrwyattii

.github/workflows/nv-transformers-v100.yml

@@ -54,7 +54,7 @@ jobs:
           git clone https://github.com/huggingface/transformers
           cd transformers
           # if needed switch to the last known good SHA until transformers@master is fixed
-          # git checkout 1cc453d33
+          git checkout 6268694e2
           git rev-parse --short HEAD
           # scipy/sklearn required for tests, using the 'dev' extra forces torch re-install
           pip install .[testing]