Remove Neptune integration references and deprecate NeptuneCallback

[qgallouedec]

[tomaarsen]

I'm not very familiar with the modern transformers deprecation approach, but do we have a nice decorator here that can inform IDE's that objects are deprecated, perhaps?

[Copilot]

Pull request overview

This PR deprecates the Neptune integration in Transformers by removing documentation references and adding deprecation warnings to the NeptuneCallback class. The callback remains functional to support existing users, but will warn them to migrate to alternative experiment tracking integrations.

Key Changes

• Added deprecation warnings (docstring and runtime) to NeptuneCallback class
• Removed Neptune from the list of supported integrations in TrainingArguments documentation
• Removed Neptune setup guide and examples from PyTorch examples README
• Removed Neptune callback references from documentation across multiple languages (English, Korean, Japanese, Chinese)

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/transformers/training_args.py	Removed "neptune" from the list of supported platforms in report_to parameter documentation (2 locations)
src/transformers/integrations/integration_utils.py	Added warnings import and deprecation warning to NeptuneCallback.__init__() with corresponding docstring warning
examples/pytorch/README.md	Removed complete Neptune integration section including installation instructions, usage examples, and environment variable documentation
docs/source/en/main_classes/callback.md	Removed NeptuneCallback reference from list of available callbacks and removed autodoc entry
docs/source/zh/main_classes/callback.md	Removed NeptuneCallback reference from list of available callbacks and removed autodoc entry (Chinese documentation)
docs/source/ko/main_classes/callback.md	Removed NeptuneCallback reference from list of available callbacks and removed autodoc entry (Korean documentation)
docs/source/ja/main_classes/callback.md	Removed NeptuneCallback reference from list of available callbacks and removed autodoc entry (Japanese documentation)

[HuggingFaceDocBuilderDev]

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