Add detection of Apple silicon to backend.py

[stiber]

Possibilities include:

• MacMetalPy
• Subsets of Pytorch or JAX.
• Some mix of stuff that uses a mix of Apple Silicon accelerated libraries that we can pick and choose from (such as described by "How to optimize data science packages in Python for Apple Silicon M1/M2")
• Some sort of compatibility layer that leverages lower level APIs; see "Apple Silicon vs NVIDIA CUDA: AI Comparison 2025, Benchmarks, Advantages and Limitations" as a start.