[Feature] Define replay dataset adapter protocols

[bsprenger]

Stack from ghstack (oldest at bottom):

• [Feature] Export replay buffer contents to external datasets #4574
• [Feature] Add a Hugging Face adapter for replay datasets #4573
• -> [Feature] Define replay dataset adapter protocols #4572

Description

• Add the ReplayDatasetExporter, ReplayDatasetImporter, and combined ReplayDatasetAdapter protocols.
• Define an interface around finite map-style datasets of TensorDictBase items, with conversion to and from native external dataset types.
• Export the protocols from torchrl.data.datasets and document them in the dataset reference.

Motivation and Context

This establishes a common contract for dataset integrations, so replay data can be exchanged with external formats without tying the core API to a specific dataset library.

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• New feature (non-breaking change which adds core functionality)
• Documentation (update in the documentation)

Checklist

• I have read the CONTRIBUTION guide (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/4572

• 📄 Preview Python docs built from this PR

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[github-actions]

⚠️ PR Title Label Error

PR title must start with a label prefix in brackets (e.g., [BugFix]).

Current title: add protocols

Supported Prefixes (case-sensitive)

Your PR title must start with exactly one of these prefixes:

Prefix	Label Applied	Example
[Algorithm]	new algo	[Algorithm] Add new RL objective
[BE]	BE	[BE] Improve error messages
[Benchmark] or [Benchmarks]	Benchmarks	[Benchmark] Add collector benchmark
[BugFix]	BugFix	[BugFix] Fix memory leak in collector
[Example] or [Examples]	Examples	[Example] Add training script
[Feature]	Feature	[Feature] Add new optimizer
[Doc] or [Docs]	Documentation	[Doc] Update installation guide
[Refactor]	Refactoring	[Refactor] Clean up module imports
[CI]	CI	[CI] Fix workflow permissions
[Test] or [Tests]	Tests	[Tests] Add unit tests for buffer
[Trainer] or [Trainers]	Trainers	[Trainer] Add trainer config
[Environment] or [Environments]	Environments	[Environments] Add Gymnasium support
[Data]	Data	[Data] Fix replay buffer sampling
[LLM]	llm/	[LLM] Add reward model integration
[Minor]	small change	[Minor] Fix typo in error message
[Performance] or [Perf]	Performance	[Performance] Optimize tensor ops
[BC-Breaking]	bc breaking	[BC-Breaking] Remove deprecated API
[Deprecation]	Deprecation	[Deprecation] Mark old function
[Algorithm] or [Algorithms]	new algo	[Algorithm] Add new objective
[Quality]	Quality	[Quality] Fix typos and add codespell
[Versioning]	versioning	[Versioning] Bump release version
[WIP]	WIP	[WIP] Draft implementation

Note: Common variations like singular/plural are supported (e.g., [Doc] or [Docs]).

[github-actions]

⚠️ PR Title Label Error

PR title must start with a label prefix in brackets (e.g., [BugFix]).

Current title: add protocols

Supported Prefixes (case-sensitive)

Your PR title must start with exactly one of these prefixes:

Prefix	Label Applied	Example
[Algorithm]	new algo	[Algorithm] Add new RL objective
[BE]	BE	[BE] Improve error messages
[Benchmark] or [Benchmarks]	Benchmarks	[Benchmark] Add collector benchmark
[BugFix]	BugFix	[BugFix] Fix memory leak in collector
[Example] or [Examples]	Examples	[Example] Add training script
[Feature]	Feature	[Feature] Add new optimizer
[Doc] or [Docs]	Documentation	[Doc] Update installation guide
[Refactor]	Refactoring	[Refactor] Clean up module imports
[CI]	CI	[CI] Fix workflow permissions
[Test] or [Tests]	Tests	[Tests] Add unit tests for buffer
[Trainer] or [Trainers]	Trainers	[Trainer] Add trainer config
[Environment] or [Environments]	Environments	[Environments] Add Gymnasium support
[Data]	Data	[Data] Fix replay buffer sampling
[LLM]	llm/	[LLM] Add reward model integration
[Minor]	small change	[Minor] Fix typo in error message
[Performance] or [Perf]	Performance	[Performance] Optimize tensor ops
[BC-Breaking]	bc breaking	[BC-Breaking] Remove deprecated API
[Deprecation]	Deprecation	[Deprecation] Mark old function
[Algorithm] or [Algorithms]	new algo	[Algorithm] Add new objective
[Quality]	Quality	[Quality] Fix typos and add codespell
[Versioning]	versioning	[Versioning] Bump release version
[WIP]	WIP	[WIP] Draft implementation

Note: Common variations like singular/plural are supported (e.g., [Doc] or [Docs]).