Fix PyTorch reduction contract edge cases

[FlorianPfaff]

Summary

• Add NumPy-compatible PyTorch reduction handling for mean/std edge cases.
• Improve reduction argument normalization and dtype behavior.
• Add backend contract coverage for the reduction semantics.

Validation

• Not run, per request.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	64.94s
✅ JSON	prettier	7	0	0	0	1.09s
✅ JSON	v8r	7		0	0	5.73s
✅ MARKDOWN	markdownlint	68	0	0	0	1.77s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.66s
✅ PYTHON	black	821	1	0	0	52.7s
✅ PYTHON	isort	821	2	0	0	3.93s
✅ REPOSITORY	checkov	yes		no	no	47.21s
✅ REPOSITORY	gitleaks	yes		no	no	12.38s
✅ REPOSITORY	git_diff	yes		no	no	0.12s
✅ REPOSITORY	secretlint	yes		no	no	22.82s
✅ REPOSITORY	syft	yes		no	no	2.03s
✅ REPOSITORY	trivy-sbom	yes		no	no	7.26s
✅ REPOSITORY	trufflehog	yes		no	no	26.61s
✅ YAML	prettier	11	0	0	0	0.68s
✅ YAML	v8r	11		0	0	10.81s
✅ YAML	yamllint	11		0	0	0.54s

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