Added Luke Doctests
# What does this PR do?

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Adds improved Doctests for LUKE

Issue: https://github.com/huggingface/transformers/issues/16292


## Before submitting
- [x] 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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- [x] Did you write any new necessary tests?


## Who can review?

@ydshieh @patrickvonplaten @patil-suraj


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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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_The documentation is not available anymore as the PR was closed or merged._<|||||>@ydshieh maybe  :-) <|||||>The checkpoints in hf-internal-testing are only there for our tests, they shouldn't be used. Please use real checkpoints in the doc examples.<|||||>@Tegzes You can search on the Hub to find if there are luke based models for the corresponding tasks. If most tasks lack such a checkpoint, we will have to skip Luke for doctest unfortunately.<|||||>> The checkpoints in hf-internal-testing are only there for our tests, they shouldn't be used. Please use real checkpoints in the doc examples.

I only used the hf-internal-testing checkpoint because these were mentioned in the comment section of the issue. But I will change them accordingly.<|||||>@ydshieh I've tried most of the luke model checkpoints and unfortunately the only ones that exhibit a fully reproducible behavior are the ones from hf-internal-testing. 

If it would be acceptable to you, I would propose the merge as it is, backed by the fact that these internal types of checkpoints were used in the past for multiple such doctests improvements, as suggested in the comments from the issue (https://github.com/huggingface/transformers/issues/16292).<|||||>@Tegzes Thank you for the effort to check the checkpoints.

We have an internal discussion, and @sgugger strongly suggests we should not have used those tiny checkpoints in the first places, and we will try to figure out what changes we should proceed.

So unfortunately, we won't merge this PR. Sorry about this.  It's possible to rework on the Luke docstrings in the future after some decision is made regarding the doctest + missing checkpoints.

However, thank you again for the contribution!<|||||>@ydshieh Thank you for the response. The situation is much clearer now

<|||||>Close as we have to decide what to do with doctest when a real checkpoint is missing