[docs] update dead quickstart link on resuing past for GPT2 (#13455)

* [docs] update dead quickstart link on resuing past for GPT2

Thed dead link have been replaced by two links of forward and call methods of the GPT2 class for torch and tensorflow respectively.

* [docs] fix formatting for gpt2 page update

docs/source/model_doc/gpt2.rst

@@ -36,10 +36,11 @@ Tips:
 - GPT-2 was trained with a causal language modeling (CLM) objective and is therefore powerful at predicting the next
   token in a sequence. Leveraging this feature allows GPT-2 to generate syntactically coherent text as it can be
   observed in the `run_generation.py` example script.
-- The PyTorch models can take the `past` as input, which is the previously computed key/value attention pairs. Using
-  this `past` value prevents the model from re-computing pre-computed values in the context of text generation. See
-  `reusing the past in generative models <../quickstart.html#using-the-past>`__ for more information on the usage of
-  this argument.
+- The model can take the `past_key_values` (for PyTorch) or `past` (for TF) as input, which is the previously computed
+  key/value attention pairs. Using this (`past_key_values` or `past`) value prevents the model from re-computing
+  pre-computed values in the context of text generation. For PyTorch, see `past_key_values` argument of the
+  :meth:`~transformers.GPT2Model.forward` method, or for TF the `past` argument of the
+  :meth:`~transformers.TFGPT2Model.call` method for more information on its usage.
 
 `Write With Transformer <https://transformer.huggingface.co/doc/gpt2-large>`__ is a webapp created and hosted by
 Hugging Face showcasing the generative capabilities of several models. GPT-2 is one of them and is available in five