[docs] XLNETModel forward returns last_hidden_state 3rd dim should be d_model instead of hidden_size 
**Link to doc** - https://huggingface.co/transformers/model_doc/xlnet.html#xlnetmodel

**Problem description** - forward method of XLNETModel returns `last_hidden_state` with dimension `(batch_size, num_predict, hidden_size)`. However, `hidden_size` is not an XLNET config (unlike for BERT) instead that dimension should be `d_model` which is a config of XLNETModel.

**current shape** - (batch_size, num_predict, hidden_size)
**expected shape** - (batch_size, num_predict, d_model)

## Environment info
Not required
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### Who can help
@sgugger
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Hi,

Actually, `hidden_size` is an alias for `d_model`, as can be seen [here](https://github.com/huggingface/transformers/blob/7e73601f3240b99d952c34b63bf4f8b78ca1462d/src/transformers/models/xlnet/configuration_xlnet.py#L233).

They mean the same thing. But I get your confusion, as some models use `hidden_size`, others `d_model`, or other names.

Feel free to open a PR to fix this in the docs of XLNet.<|||||>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.