Add doc about `attention_mask` on gpt2 
Author: wiio12
Pull request: yes

# What does this PR do?

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Fixes #16811. 

>If `past_key_values` is used, `attention_mask` needs to contain the masking strategy that was used for  `past_key_values`. In other words, the attention_mask always has to have the length:  `len(past_key_values) + len(input_ids)`

I add the sentence above describing how `attention_mask` needs to be constructed when `past_key_values` is used. The sentence is added in both the `PyTorch` and  `TensorFlow` versions of code.


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