Fix PPLM
Author: chutaklee
Pull request: yes

# What does this PR do?

API changes break PPLM example, this PR should fix it. However I haven't test it on 'run_pplm_discrim_train.py'.

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Comments:
PPLM is unfortunately not maintained anymore. The fix would be to pin the `transformers` version in the PPLM README.

pinging @w4nderlust and @mimosavvy just in case

This looks good to me, I believe the return of dict (very welcome change by the way) from the model should be the only thing breaking the pplm code, right?

> This looks good to me, I believe the return of dict (very welcome change by the way) from the model should be the only thing breaking the pplm code, right?

Yeah and the named argument of model `past_key_values`. I have no issue running the provided example command in [here]( https://github.com/huggingface/transformers/tree/master/examples/text-generation/pplm) and `python run_pplm_discrim_train.py --dataset SST --epochs 1 --batch_size 8`.

Can you run `make style` to  fix the code quality check? Then we should be good for merge :-) 

Much appreciated!