[BugFix] Fix kwargs dropping in AsyncEnvPool.reset

[coder-jayp]

Description

This PR addresses the follow-up issue where AsyncEnvPool.reset drops **kwargs before they reach the sub-environments, which caused features like set_state=True to fail silently.

Fixes #4520

Changes:

• Safely forwarded **kwargs through AsyncEnvPool.reset to async_reset_send.
• Appended kwargs into the worker queue tuples inside _send_worker_batches.
• Gracefully unpacked varying length tuples inside the multiprocessing _worker_exec and _env_exec (providing a {} fallback for legacy/internal messages like step or init_shm to guarantee zero regressions).
• Passed kwargs directly into env.reset(**kwargs).
• Replicated the identical logic for the ThreadPoolExecutor backend in ThreadingAsyncEnvPool.
• Added a formal regression test test_async_pool_kwargs_forwarding to ensure kwargs are always forwarded.

Motivation and Context

The maintainer requested a proper forwarding mechanism in a follow-up PR to #4501. This implementation relies strictly on native tuple forwarding/unpacking to achieve cross-process boundary communication with absolutely zero API breakage or heavy abstraction changes.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/4521

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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: Fix kwargs dropping in AsyncEnvPool.reset

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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: Fix kwargs dropping in AsyncEnvPool.reset

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]).

[coder-jayp]

@torchrlbot reviewer @theap06

[github-actions]

Requested review from @theap06 (requested by @coder-jayp).