Add nltk to ALBERT example's requirements (#251)

nltk is used by the `tokenize_wikitext103.py` script but is missing in the requirements.

examples/albert/requirements.txt

@@ -4,3 +4,4 @@ torch_optimizer>=0.1.0
 wandb>=0.10.26
 sentencepiece
 whatsmyip
+nltk>=3.6.2