added doctring to SchedulerType class
Author: Arunprakash-A
Pull request: yes

# What does this PR do?
It adds the docstring for the class `SchedulerType` . Currently, the AutoDoc only displays the `__call__ ` signature of `enum.EnumType` base class which is not helpful. One needs to look for the names and the respective mapping in the source code of trainer.py. Therefore, added the docstring that lists out all the available names and the scheduler types they are mapped to.
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## Before submitting
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Documentation: @stevhliu



Comments:
@stevhliu i ran `make style` in the command line. It catched style errors in other documents, for example, "examples/research_projects/movement-pruning/Saving_PruneBERT.ipynb". I am not sure whether those should be fixed before making this pull request. 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_32898). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

@stevhliu @Arunprakash-A For the check_repo_consistency tests - the tests were failing because of packages on the test image. These should have been resolved now. Pushing an empty commit to trigger a new CI run should resolve. 

For `check_code_quality` - there's a formatting issue and you should run `make fixup` and push the changes to resolve 

> @stevhliu @Arunprakash-A For the check_repo_consistency tests - the tests were failing because of packages on the test image. These should have been resolved now. Pushing an empty commit to trigger a new CI run should resolve.
> 
> For `check_code_quality` - there's a formatting issue and you should run `make fixup` and push the changes to resolve

I ran `make fixup`  and got the following
```
Checking/fixing src/transformers/trainer_utils.py
All checks passed!
1 file reformatted

python utils/custom_init_isort.py
python utils/sort_auto_mappings.py
python utils/check_doc_toc.py --fix_and_overwrite
running deps_table_update
updating src/transformers/dependency_versions_table.py
python utils/check_copies.py
python utils/check_table.py
python utils/check_dummies.py
python utils/check_repo.py
<truncated>:
```

@Arunprakash-A You have to commit and push the changes made when running `make fixup`

> @Arunprakash-A You have to commit and push the changes made when running `make fixup`

Thanks @amyeroberts. Done!