docs: adding horovod

[Borda]

What does this PR do?

adding ingo about HorovodStrategy, https://github.com/Lightning-AI/lightning-Horovod

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• Was this discussed/agreed via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

Anyone in the community is welcome to review the PR.
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Reviewer checklist

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cc @Borda

[github-actions]

⛈️ Required checks status: Has failure 🔴

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Groups summary

🔴 pytorch_lightning: Tests workflow

Check ID	Status
pl-cpu (macOS-11, lightning, 3.8, 1.11)	success	✅
pl-cpu (macOS-11, lightning, 3.9, 1.12)	success	✅
pl-cpu (macOS-11, lightning, 3.10, 1.13)	success	✅
pl-cpu (macOS-11, lightning, 3.8, 1.11, oldest)	success	✅
pl-cpu (macOS-11, lightning, 3.9, 2.0, pre)	success	✅
pl-cpu (ubuntu-20.04, lightning, 3.9, 1.11)	success	✅
pl-cpu (ubuntu-20.04, lightning, 3.10, 1.12)	success	✅
pl-cpu (ubuntu-20.04, lightning, 3.10, 1.13)	success	✅
pl-cpu (ubuntu-20.04, lightning, 3.8, 1.11, oldest)	success	✅
pl-cpu (ubuntu-20.04, lightning, 3.9, 2.0, pre)	success	✅
pl-cpu (windows-2022, lightning, 3.9, 1.11)	failure	❌
pl-cpu (windows-2022, lightning, 3.10, 1.12)	success	✅
pl-cpu (windows-2022, lightning, 3.10, 1.13)	success	✅
pl-cpu (windows-2022, lightning, 3.8, 1.11, oldest)	success	✅
pl-cpu (windows-2022, lightning, 3.9, 2.0, pre)	success	✅
pl-cpu (macOS-11, pytorch, 3.8, 1.13)	success	✅
pl-cpu (ubuntu-20.04, pytorch, 3.8, 1.13)	success	✅
pl-cpu (windows-2022, pytorch, 3.8, 1.13)	success	✅

These checks are required after the changes to requirements/pytorch/strategies.txt, src/lightning/pytorch/callbacks/gradient_accumulation_scheduler.py, src/lightning/pytorch/trainer/connectors/accelerator_connector.py, src/lightning/pytorch/utilities/imports.py.

🔴 pytorch_lightning: Azure GPU

Check ID	Status
pytorch-lightning (GPUs)	failure	❌

These checks are required after the changes to requirements/pytorch/strategies.txt, src/lightning/pytorch/callbacks/gradient_accumulation_scheduler.py, src/lightning/pytorch/trainer/connectors/accelerator_connector.py, src/lightning/pytorch/utilities/imports.py.

🔴 pytorch_lightning: Benchmarks

Check ID	Status
pytorch-lightning.Benchmark	failure	❌

These checks are required after the changes to requirements/pytorch/strategies.txt.

🟢 pytorch_lightning: Azure HPU

Check ID	Status
pytorch-lightning (HPUs)	success	✅

These checks are required after the changes to requirements/pytorch/strategies.txt, src/lightning/pytorch/callbacks/gradient_accumulation_scheduler.py, src/lightning/pytorch/trainer/connectors/accelerator_connector.py, src/lightning/pytorch/utilities/imports.py.

🟢 pytorch_lightning: Docs

Check ID	Status
make-doctest (pytorch)	success	✅
make-html (pytorch)	success	✅

These checks are required after the changes to src/lightning/pytorch/callbacks/gradient_accumulation_scheduler.py, src/lightning/pytorch/trainer/connectors/accelerator_connector.py, src/lightning/pytorch/utilities/imports.py, docs/source-pytorch/advanced/model_parallel.rst, docs/source-pytorch/advanced/third_party/horovod.rst, docs/source-pytorch/extensions/strategy.rst, requirements/pytorch/strategies.txt.

🔴 pytorch_lightning: Docker

Check ID	Status
build-cuda (3.9, 1.11, 11.3.1)	success	✅
build-cuda (3.9, 1.12, 11.6.1)	success	✅
build-cuda (3.9, 1.13, 11.7.1)	success	✅
build-cuda (3.10, 1.13, 11.7.1)	success	✅
build-hpu (1.5.0, 1.11.0)	success	✅
build-ipu (3.9, 1.13)	success	✅
build-NGC	in_progress	⌛
build-pl (3.9, 1.11, 11.3.1)	failure	❌
build-pl (3.9, 1.12, 11.6.1)	failure	❌
build-pl (3.9, 1.13, 11.7.1)	failure	❌
build-xla (3.8, 1.12)	success	✅

These checks are required after the changes to requirements/pytorch/strategies.txt.

🟢 mypy

Check ID	Status
mypy	success	✅

These checks are required after the changes to requirements/pytorch/strategies.txt, src/lightning/pytorch/callbacks/gradient_accumulation_scheduler.py, src/lightning/pytorch/trainer/connectors/accelerator_connector.py, src/lightning/pytorch/utilities/imports.py.

🟢 install

Check ID	Status
install-pkg (ubuntu-22.04, app, 3.8)	success	✅
install-pkg (ubuntu-22.04, app, 3.10)	success	✅
install-pkg (ubuntu-22.04, fabric, 3.8)	success	✅
install-pkg (ubuntu-22.04, fabric, 3.10)	success	✅
install-pkg (ubuntu-22.04, pytorch, 3.8)	success	✅
install-pkg (ubuntu-22.04, pytorch, 3.10)	success	✅
install-pkg (ubuntu-22.04, lightning, 3.8)	success	✅
install-pkg (ubuntu-22.04, lightning, 3.10)	success	✅
install-pkg (ubuntu-22.04, notset, 3.8)	success	✅
install-pkg (ubuntu-22.04, notset, 3.10)	success	✅
install-pkg (macOS-12, app, 3.8)	success	✅
install-pkg (macOS-12, app, 3.10)	success	✅
install-pkg (macOS-12, fabric, 3.8)	success	✅
install-pkg (macOS-12, fabric, 3.10)	success	✅
install-pkg (macOS-12, pytorch, 3.8)	success	✅
install-pkg (macOS-12, pytorch, 3.10)	success	✅
install-pkg (macOS-12, lightning, 3.8)	success	✅
install-pkg (macOS-12, lightning, 3.10)	success	✅
install-pkg (macOS-12, notset, 3.8)	success	✅
install-pkg (macOS-12, notset, 3.10)	success	✅
install-pkg (windows-2022, app, 3.8)	success	✅
install-pkg (windows-2022, app, 3.10)	success	✅
install-pkg (windows-2022, fabric, 3.8)	success	✅
install-pkg (windows-2022, fabric, 3.10)	success	✅
install-pkg (windows-2022, pytorch, 3.8)	success	✅
install-pkg (windows-2022, pytorch, 3.10)	success	✅
install-pkg (windows-2022, lightning, 3.8)	success	✅
install-pkg (windows-2022, lightning, 3.10)	success	✅
install-pkg (windows-2022, notset, 3.8)	success	✅
install-pkg (windows-2022, notset, 3.10)	success	✅

These checks are required after the changes to src/lightning/pytorch/callbacks/gradient_accumulation_scheduler.py, src/lightning/pytorch/trainer/connectors/accelerator_connector.py, src/lightning/pytorch/utilities/imports.py, requirements/pytorch/strategies.txt.

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