Add missing optional argument summary_proj_to_labels to XLNetConfig
# What does this PR do?

`XLNetConfig` (`src\transformers\models\xlnet\configuration_xlnet.py`) lists an argument `summary_proj_to_labels` as optional and with a default value of `True`. However, this is not actually included in the arguments and is not set anywhere. Initializing an XLNet model thus results in no such parameter existing. Also fixes a very minor typo (`boo` -> `bool`).

For reference: the same argument also is utilized in `XLMConfig` (`src\transformers\models\xlm\configuration_xlm.py`) but is actually utilized there.

From personal experience, this argument is used in `SequenceSummary` (`src\transformers\modeling_utils.py`). When using default arguments, there is an inconsistency between the models where one would have `hidden_size` -> `hidden_size` layers while the other would have `hidden_size` -> `num_labels` layers. 


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

cc: @sgugger 

Still a draft right now. Need to make changes to the tests to adapt to this. Please let me know if this is intended functionality though.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_22020). All of your documentation changes will be reflected on that endpoint.<|||||>Yes. I wanted to get some clarity on that before working on fixing the tests. From a logical perspective, I would believe that the argument should exist since at the end of the day it decides only one thing - "Whether the projection outputs should have config.num_labels or config.hidden_size classes.". As it stands, it will only use `config.hidden_size` classes since the argument doesn't currently exist. Do you know someone who might be able to clarify this?

I believe the tests are failing since this change causes the output of the summary layer to be different. Setting `summary_proj_to_labels=False` causes all tests to pass locally. I'll look into the tests sometime this weekend assuming that we want to include this change.<|||||>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.

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