GPT2 Generate doesn't pass the user defined past_key_values.
## Environment info
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- `transformers` version:
- Platform: MacOS Intel
- Python version: 3.7.10
- PyTorch version (GPU?): '1.8.1' (CPU)
- Tensorflow version (GPU?): '1.15.0' (CPU)
- Using GPU in script?: Nope
- Using distributed or parallel set-up in script?: Nope

### Who can help
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 @patrickvonplaten, @LysandreJik, @cccntu  

Models:

- ALBERT, BERT, XLM, DeBERTa, DeBERTa-v2, ELECTRA, MobileBert, SqueezeBert: @LysandreJik
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- Longformer, Reformer, TransfoXL, XLNet, FNet: @patrickvonplaten
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- GPT-2, GPT: @patrickvonplaten, @LysandreJik
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- JAX/Flax: @patil-suraj @patrickvonplaten 
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- Wav2Vec2, HuBERT, SpeechEncoderDecoder: @patrickvonplaten, @anton-l

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- Benchmarks: @patrickvonplaten
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- Pipelines: @Narsil
- Speech: @patrickvonplaten, @anton-l
- Vision: @NielsRogge, @sgugger

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## Information

Bug in Huggingface Transformers "generate" for Auto-Regression Model (like GPT-2). If you wanna pass your own "past_key_values", the function will not pass it to the model as you wish. 

## To reproduce

Send used defined "past_key_values" to the generate() function, the "past_key_values" in model forward() should be none.



This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Hey @KnightZhang625, 

Could you please provide a reproducible code-snippet?<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.