Fix PyTorch concatenate axis=None contract

[FlorianPfaff]

Summary

• patch the PyTorch backend compatibility layer so concatenate(..., axis=None) flattens inputs before calling torch.cat, matching NumPy semantics
• add regression coverage for both the public PyTorch backend facade and the raw PyTorch backend import path

Fixes #3677.

Testing

• Not run locally: this environment cannot clone or install the repository from GitHub directly.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	78.33s
✅ JSON	prettier	7	0	0	0	1.0s
✅ JSON	v8r	7		0	0	5.06s
✅ MARKDOWN	markdownlint	68	0	0	0	1.8s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.86s
✅ PYTHON	black	1204	35	0	0	66.02s
✅ PYTHON	isort	1204	68	0	0	4.96s
✅ REPOSITORY	checkov	yes		no	no	48.48s
✅ REPOSITORY	gitleaks	yes		no	no	12.17s
✅ REPOSITORY	git_diff	yes		no	no	0.2s
✅ REPOSITORY	secretlint	yes		no	no	41.76s
✅ REPOSITORY	syft	yes		no	no	4.64s
✅ REPOSITORY	trivy-sbom	yes		no	no	7.39s
✅ REPOSITORY	trufflehog	yes		no	no	21.37s
✅ YAML	prettier	11	0	0	0	0.75s
✅ YAML	v8r	11		0	0	11.76s
✅ YAML	yamllint	11		0	0	0.56s

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