add requirements and fix minor bug in gpt2 example

[LauraRuis]

Just adding some requirements needed for running wiki example + fix a minor bug in train.py

[sangkeun00]

Hi Laura,

Thanks for fixing the bug in our GPT training code.
Regarding the requirements, dependencies like transformers and accelerate are specific to some examples (e.g. BERT, GPT), but not to analog itself. Therefore, I am inclined to include these dependencies in a separate requirements.txt in each example directory. Any thoughts?

[eatpk]

I've tried to address this problem here, PTAL!
#89

[sangkeun00]

Fixed in 21a5168