[TFXLNet] Correct tf xlnet generate
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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The TF XLNet integration test (https://github.com/huggingface/transformers/blob/35ecf99cc4e1046e5908f5933a4548e43a0f4f1d/tests/xlnet/test_modeling_tf_xlnet.py#L845) was not in-line with the PT XLNet integration test (https://github.com/huggingface/transformers/blob/35ecf99cc4e1046e5908f5933a4548e43a0f4f1d/tests/xlnet/test_modeling_xlnet.py#L1058) probably because our previous TF generate method incorrectly did **not** forward the `mems` (=XLNet's `past_key_value` states) during generation whereas the PT generate method does. 

Since the TF generate refactor: https://github.com/huggingface/transformers/pull/15562 the generation method was correctly implemented however which now allows us to align the tests.


## 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.

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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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Comments:
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