13. Add explicit device validation and fallback handling

[opeyemibami]

Description

VAASPipeline currently assumes the provided device string is valid.
Invalid or unavailable devices can lead to unclear runtime errors.

This issue proposes adding explicit device validation and clearer error handling.

Scope

• Validate device string at pipeline creation
• Provide meaningful error messages or fallback behavior
• Preserve current default behavior (cpu)

Acceptance Criteria

• Invalid devices fail with clear, user-facing errors
• Valid devices behave unchanged
• No silent device misconfiguration

Notes / Constraints

• Must not introduce new dependencies
• Must preserve lazy loading of torch

Testing Requirements

• Add tests for valid and invalid device inputs
• Ensure error messages are informative