Fix tapas scatter
# What does this PR do?

Changes the usage of scatter from torch_scatter to PyTorch's scatter thus removing the dependency on third party library

* [x] remove torch_scatter dependency
* [x] update  `_segment_reduce` function in order to work with PyTorch's scatter
* [x] update test case `test_reduce_sum_vectorized`



Fixes # (issue)
https://github.com/huggingface/transformers/issues/20101

## Who can review?

@NielsRogge 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20149). All of your documentation changes will be reflected on that endpoint.<|||||>Hi @sgugger - Sure, I will remove it as a part of removing all "scatter" mentions requested by @NielsRogge.<|||||>Hi @NielsRogge - could you tell me what is the difference between `require_scatter` and `require_torch_scatter` in `transformers/src/transformers/testing_utils.py` since they are calling the same thing.<|||||>Good question, you can remove both ;)<|||||>@NielsRogge - I have removed "scatter" mentions from the code base. It will be good to double check the changes :). I have not changed `.circleci/create_circleci_config.py` since removing it is a part of the #20168 PR.<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20149). All of your documentation changes will be reflected on that endpoint.<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20149). All of your documentation changes will be reflected on that endpoint.<|||||>Thanks for all the work! Will rebase my PR on yours to finish the job :-)