Typo in GPT2 documentation 
# 🐛 Bug

In the GPT2 documentation [page](https://huggingface.co/transformers/model_doc/gpt2.html#gpt2lmheadmodel), in the parameters of both LMHead and DoubleHeads model, 'inputs_embeds' is registered as 'input_embeds' which leads to an error upon implementation.

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