fix #2399 an ImportError in official example (#2400)
fix #2399 an ImportError in official example (#2400)

* fix #2399 an ImportError in official example

* style

Co-authored-by: Julien Chaumond <a2e75dc0f7a441a5f83dcee1a3f7492a8c3d1d78@gmail.com>

huggingface/transformers,huggingface/transformers,huggingface/transformers,huggingface/pytorch-transformers,huggingface/transformers