Add MPS backend for Apple Silicon

[imperatormk]

Summary

• Adds native MPS (Metal Performance Shaders) backend for Apple Silicon Macs
• Implements 8-bit optimizers (Adam, RMSprop, Lion, Momentum, AdEMAMix) using dynamic codebook quantization
• Adds INT8 and 4-bit (NF4/FP4) quantization and matrix multiplication support
• Updates README to reflect full MPS support
• Enables MPS device in 8-bit optimizer test suite

Dependencies

Requires mps-bitsandbytes for Metal kernels:

pip install mps-bitsandbytes

Implementation Details

• New bitsandbytes/backends/mps/ module with ops registration
• Uses torch.library.register_kernel for seamless integration with existing torch.ops.bitsandbytes calls
• Dynamic codebook quantization matching CUDA behavior (256 values, blockwise absmax scaling)
• Falls back gracefully when mps_bitsandbytes package is not installed

Registered kernels:

• 4-bit: quantize_4bit, dequantize_4bit, gemv_4bit
• 8-bit blockwise: quantize_blockwise, dequantize_blockwise
• INT8: int8_linear_matmul, int8_mm_dequant, int8_vectorwise_quant, int8_vectorwise_dequant, int8_scaled_mm
• Optimizers: optimizer_update_8bit_blockwise (Adam, Momentum, RMSprop, Lion, AdEMAMix)

Notes

12 bf16/fp16 8-bit optimizer test failures:

• bf16 adam/momentum/rmsprop/ademamix: relerr 0.0017-0.002 (threshold 0.0016)
• fp16 rmsprop: relerr 0.0007-0.0008 (threshold 0.0006)