Give `modeling_t5.py` a `_prune_heads`
# What does this PR do?

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Fixes # (issue) 19960 (not perfectly)
I refer a colab script in issue, it can prune but with forward problems. 
You can see here https://colab.research.google.com/drive/1b9mHjtn2UxuHU_Sb_RXts12rDzbebBX0#scrollTo=hUSe4a1oOp6D


I use `opendelta` to visualize the  pruning process.

But we seems to be a forward problem


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
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- [x] Did you write any new necessary tests?


## Who can review?

@ArthurZucker

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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

 -->

Great work! Fell free to ping me for a review 👍<|||||>Hi @ArthurZucker , please pay more attention to the `position_bias`, I think I change it too sharply otherwise the shape will not be the same to the `score`<|||||>Hi @ArthurZucker , I upload a new commits, it seems better deal with `position_bias`. And if we do not add `head_mask` and `decoder_head_mask` in the model `forward`, the code can run. But we just ignore this line's problem.

https://github.com/huggingface/transformers/blob/c3a93d8d821bc1df3601ba858e7385eada8db3a5/src/transformers/models/t5/modeling_t5.py#L547



<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19975). All of your documentation changes will be reflected on that endpoint.<|||||>Okay! That's way better, the least code we change the better. I remember seeing a similar fix, let me link that and I think we will be able to merge if slow tests pass 👍<|||||>Great @ArthurZucker ! What other tests should be done? <|||||>gently pin @patrickvonplaten Thanks<|||||>Hey, the tests are simply the CircleCI tests! Try running `make fixup` and `make fix-copies`. The integrations tests have to pass to be able to merge the PR! <|||||>Hi @ArthurZucker  I have try to pull new request after `make fixup` and `make fix-copies`, but it still can not pass CircleCI :(<|||||>Hey! You didn't have to open a new PR! I will have a look and help you fix the tests ☺️<|||||>OK then ! Thanks! It seems my flat can not work on the `make fixup`<|||||>Okay, it seems that #19097 also wanted to adress part of this issue. Since it has not really progressed, we can do everything here<|||||>Great! Anything that I can help?
<|||||>OKay, try running : 
- `make style` to pass the `check_code_quality` 
- `git pull upstream main` to merge the changes from the `huggingface/transformers/main` branch. 
Then we will try to debug the tests that are failing. 
You can also do this by running `RUN_SLOW=1 pytest tests/models/t5/test_modeling_t5.py`. <|||||>Hi @ArthurZucker ,I try to `make style`, it reports error ! And this commits seems unsuccessful
```
All done! ✨ 🍰 ✨
597 files reformatted, 1299 files left unchanged.
isort examples tests src utils
Skipped 1 files
/Library/Developer/CommandLineTools/usr/bin/make autogenerate_code
running deps_table_update
updating src/transformers/dependency_versions_table.py
/Library/Developer/CommandLineTools/usr/bin/make extra_style_checks
python utils/custom_init_isort.py
python utils/sort_auto_mappings.py
doc-builder style src/transformers docs/source --max_len 119 --path_to_docs docs/source
make[1]: doc-builder: No such file or directory
make[1]: *** [extra_style_checks] Error 1
make: *** [style] Error 2

```<|||||>Maybe it is my machine's problem of `make style`? @ArthurZucker  Macbook pro with m1 maybe? I could not install `doc-builder`
<|||||>No, the error is from the missing `huggingface_doc` package ! Don't worry. Try installing it. 
The files should still have been formatted <|||||>OK! I install it and rerun `make style` and `python utils/check_copies.py --fix_and_overwrite`
Hope it will work this time<|||||>Hi @ArthurZucker it seems that `make style` can run well on linux but cannot run well in macOS system. :) Maybe it is better to find a new method for Apple M1 :)<|||||>Hi @ArthurZucker , could you please give me a review? Many thanks!<|||||>Hi @ArthurZucker  
<|||||>Hey, let's try to rebase to `be59316681fca13483da0ac2eac341f7df090e35`, since a loooot of files were modified by make style ( and this is not normal!). The issue most probably comes from your version of `black.  `pip install hf-doc-builder` or upgrading it sould solve this! 
I will review once that's clean! I can also help with make style if you are still unable to have the expected result! 🤗  <|||||>Hi @ArthurZucker , what is rebase? BTW? Could you please help me make style? Since my machine's version is too tricky<|||||>Let me re-fork the link it seems too dirty at this point