[codex] Fix PyTorch backend std signature

[FlorianPfaff]

Summary

• Replace the PyTorch backend std wrapper with a NumPy-compatible signature for axis, ddof, keepdims, dtype, out, and correction.
• Add focused PyTorch backend tests covering population vs. sample standard deviation, keepdims, dtype, and conflicting ddof/correction arguments.

Notes

This intentionally excludes the SE2PartiallyWrappedNormalDistribution alias and other unrelated changes from #1807.

Validation

• Not run locally; this workspace is read-only and the branch was created through the GitHub API.
• Verified the GitHub compare contains only src/pyrecest/_backend/pytorch/__init__.py and tests/test_pytorch_backend.py.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	16.63s
✅ JSON	prettier	2	0	0	0	0.44s
✅ JSON	v8r	2		0	0	2.86s
✅ MARKDOWN	markdownlint	2	0	0	0	0.69s
✅ MARKDOWN	markdown-table-formatter	2	0	0	0	0.26s
✅ PYTHON	bandit	394		0	0	5.99s
✅ PYTHON	black	394	6	0	0	9.24s
✅ PYTHON	flake8	394		0	0	3.37s
✅ PYTHON	isort	394	10	0	0	0.76s
✅ PYTHON	mypy	394		0	0	5.34s
✅ PYTHON	pylint	394		0	0	120.52s
✅ PYTHON	ruff	394	10	0	0	0.08s
✅ REPOSITORY	checkov	yes		no	no	23.87s
✅ REPOSITORY	gitleaks	yes		no	no	11.32s
✅ REPOSITORY	git_diff	yes		no	no	0.06s
✅ REPOSITORY	secretlint	yes		no	no	6.97s
✅ REPOSITORY	syft	yes		no	no	2.83s
✅ REPOSITORY	trivy-sbom	yes		no	no	3.07s
✅ REPOSITORY	trufflehog	yes		no	no	20.46s
✅ YAML	prettier	4	0	0	0	0.49s
✅ YAML	v8r	4		0	0	4.78s
✅ YAML	yamllint	4		0	0	0.45s

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