Changed the hidden_size to d_model for XLNET docs
Author: Muktan
Pull request: yes

# What does this PR do?
Changes the use of hidden_size to d_model as XLNET model (unlike BERT and other) uses `d_model` instead of `hidden_size`.

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Fixes #11938 


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).


## Who can review?
@sgugger @NielsRogge 

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Comments:
Many models use different names for this and `hidden_size` is a constant property that works for all of them, so we will keep the docstrings updated with this. It also simplifies copy-pasting the docstrings between models.

> Many models use different names for this and `hidden_size` is a constant property that works for all of them, so we will keep the docstrings updated with this. It also simplifies copy-pasting the docstrings between models.

Okay, in that case at least we can mention somewhere in the documentation of the XLNET that hidden_size is same as d_model. Because for someone who starts working with XLNET and has not worked with other models won't understand what is hidden_size, as hidden_size is not described in XLNET docs and is mentioned as one of the dimensions in the output. 

How about adding an entry in the glossary page, along with input IDs, attention mask etc?

> How about adding an entry in the glossary page, along with input IDs, attention mask etc?

along with.. means inside those sections [input IDs section, attention mask section] or creating another section for it?

If thinking of adding it to the glossary we can write a line in docs of XLNET, BERT ..(other models) that explanation of some terms can be explored in the glossary. As all the documentations (pandas, pytorch... etc) don't have a glossary section and it can't be expected from the reader that he/she will search for glossary section if they encounter any term not explain in a documentation page.



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