[BugFix] Fix hidden multiprocessing import in coding_ppo tutorial

[coder-jayp]

Description

Moved the from torch import multiprocessing import outside the sphinx_gallery_start_ignore section in tutorials/sphinx-tutorials/coding_ppo.py so that it is visible in the rendered documentation.

Motivation and Context

This was a documentation bug. The tutorial was not self-contained. Readers who copied the visible code from the documentation would encounter a NameError because the import was hidden.

This fix makes the tutorial reproducible when users copy code directly from the documentation.

Fixes #3778

Types of changes

What types of changes does your code introduce?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of examples)

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/3779

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

⏳ 7 Pending, 1 Unrelated Failure

As of commit d198aae with merge base 7155a0b ():

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Libs Tests on Linux / unittests-gym (3.10, 12.8) / linux-job (gh) (trunk failure)
  test/libs/test_gym.py::TestGym::test_gym_fake_td[True-False-3-Pong-v4]

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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 hidden multiprocessing import in coding_ppo tutorial

Supported Prefixes (case-sensitive)

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

Prefix	Label Applied	Example
[BugFix]	BugFix	[BugFix] Fix memory leak in collector
[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
[Environment] or [Environments]	Environments	[Environments] Add Gymnasium support
[Data]	Data	[Data] Fix replay buffer sampling
[Performance] or [Perf]	Performance	[Performance] Optimize tensor ops
[BC-Breaking]	bc breaking	[BC-Breaking] Remove deprecated API
[Deprecation]	Deprecation	[Deprecation] Mark old function
[Quality]	Quality	[Quality] Fix typos and add codespell

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 hidden multiprocessing import in coding_ppo tutorial

Supported Prefixes (case-sensitive)

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

Prefix	Label Applied	Example
[BugFix]	BugFix	[BugFix] Fix memory leak in collector
[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
[Environment] or [Environments]	Environments	[Environments] Add Gymnasium support
[Data]	Data	[Data] Fix replay buffer sampling
[Performance] or [Perf]	Performance	[Performance] Optimize tensor ops
[BC-Breaking]	bc breaking	[BC-Breaking] Remove deprecated API
[Deprecation]	Deprecation	[Deprecation] Mark old function
[Quality]	Quality	[Quality] Fix typos and add codespell

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

[vmoens]

LGTM thanks