[Generate Tests] Make sure no tokens are force-generated
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Bart-like models often have the default of forcing certain tokens to be generated (see https://github.com/huggingface/transformers/blob/91c4a3ab1a7f6651bbcd27ccd98d8f3e69189911/src/transformers/models/bart/configuration_bart.py#L141) which could be the reason for the flaky behavior of certain generation tests - e.g. if `eos_token_id` is already `-inf` and then all other tokens are set to `-inf` because at `max_length` EOS should be generated then the test will fail. 

Would like to give this a try before tweaking the general test architecture (cc @ydshieh @sgugger )


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
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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
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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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