Small changes in `perplexity.rst`to make the notebook executable on google collaboratory
# What does this PR do?

Small changes in `perplexity.rst`to make the notebook executable on google collaboratory.

Replace this [PR](https://github.com/huggingface/notebooks/pull/85) in the `notebooks` repository.
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## 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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## Who can review?

Anyone in the community is free to review the PR once the tests have passed. @sgugger, I would love to have your thoughs on this PR.

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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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I take the opportunity to point out that a change was already made in this file and not in the corresponding file into `notebooks` repository: `from nlp import load_dataset` -> `from datasets import load_dataset`. 

Whereas when we press the `open in Colab` button in the [documentation](https://huggingface.co/transformers/perplexity.html), we get the notebook stored into the `notebooks` repository.  <|||||>> Thanks for fixing! Will upload the updated notebooks after this is merged.

With pleasure it was really a small change! To understand the workflow, what we have to do is edit the files in the `docs/source` folder, generate automatically the notebooks from these files and then upload manually the modified notebooks in the `notebooks` repository? :blush: <|||||>Yes, the exact workflow is to have an up to do date repo of transformers in the same folder has the notebooks repo, then in the notebooks repo, make sure we are up to date with master, run `make doc-notebooks`, commit and push the changes.