Wrong layer names for selecting parameters groups (run_openai_gpt.py)
Hi,
In this script [run_openai_gpt.py](https://github.com/huggingface/pytorch-transformers/blob/master/examples/single_model_scripts/run_openai_gpt.py)

Parameter names for selecting param groups are wrong.
Should be:

`  
    no_decay = no_decay = ['bias', 'ln_1.bias', 'ln_1.weight', 'ln_2.bias', 'ln_2.weight']
`
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