raise exception when arguments to pipeline are incomplete
Author: hwijeen
Pull request: yes

# What does this PR do?

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Fixes #12478  (issue)
As discussed in the issue, this PR adds an exception when arguments to `pipeline` are incomplete. Incomplete cases are providing `tokenizer` or `feature_extractor` without specifying the model, which could lead to unexpected behavior demonstrated in the issue.

## 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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- [ ] Did you write any new necessary tests?


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@LysandreJik @Narsil 

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
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Library:

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- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
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