Add PyTorch reduction backend wrappers

[FlorianPfaff]

Summary

• add NumPy-style PyTorch backend wrappers for quantile and count_nonzero
• support tuple-axis reductions and keepdims behavior for those wrappers
• add PyTorch backend regression coverage for the new reduction semantics

Validation

• git diff --check
• git diff --cached --check
• Tests not run per request.

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	36.49s
✅ JSON	prettier	7	0	0	0	0.94s
✅ JSON	v8r	7		0	0	4.48s
⚠️ MARKDOWN	markdownlint	66	0	2	0	1.65s
✅ MARKDOWN	markdown-table-formatter	66	1	0	0	0.8s
✅ PYTHON	black	758	3	0	0	34.39s
✅ PYTHON	isort	758	3	0	0	2.05s
✅ REPOSITORY	checkov	yes		no	no	41.12s
✅ REPOSITORY	gitleaks	yes		no	no	15.67s
✅ REPOSITORY	git_diff	yes		no	no	0.12s
✅ REPOSITORY	secretlint	yes		no	no	22.74s
✅ REPOSITORY	syft	yes		no	no	4.58s
✅ REPOSITORY	trivy-sbom	yes		no	no	5.0s
✅ REPOSITORY	trufflehog	yes		no	no	21.29s
✅ YAML	prettier	10	0	0	0	0.73s
✅ YAML	v8r	10		0	0	9.42s
✅ YAML	yamllint	10		0	0	0.47s

Detailed Issues

⚠️ MARKDOWN / markdownlint - 2 errors

.github/pull_request_template.md:1 error MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "## Summary"]
docs/backend-support.md:11:174 error MD060/table-column-style Table column style [Table pipe does not align with header for style "aligned"]

Notices

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