Add Metal backend

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Add Metal Backend

Implement a Metal backend for the candy framework, enabling high-performance computation leveraging Apple GPUs on macOS/iOS platforms. This backend should support tensor operations, neural network layers, and efficient data movement between host and device. Focus on extensibility and maintainability to support future Metal features and ML API integration.

Tasks

• Design API for Metal backend integration with candy
• Implement tensor operations (allocation, transfer, arithmetic, reduction) using Metal
• Support basic neural network layers and operator primitives
• Ensure interoperability and backend management with existing framework architecture
• Add error handling, resource management, and documentation
• Develop tests and benchmarks for validation

Reference: Review Metal/ML integration strategies in comparable frameworks and leverage candy's backend abstraction for seamless interop.