fix(metal): support argsort for arrays >1024 elements

[DumbTechLion]

Here is an attempt for #2570. Our ML research project requires it to work. Don't hesitate to comment or contribute in any way.

Summary

• Route large arrays (>1024 elements) to MLX multi-block merge sort instead of bitonic sort, which was limited by Metal's 1024 thread
  per threadgroup limit
• Add GreaterThan comparator for descending sort support in MLX kernels
• Add CUDA shared memory validation with clear error message when exceeding ~12K element limit
• Enable asort_big test on Metal, add vocabulary-size tests (2048, 4096, 32000 elements)

Fixes #2570

Problem

The Metal argsort kernel used bitonic sort with ncols_pad as threadgroup size. Since Metal limits threadgroups to 1024 threads,
arrays with >1024 elements failed silently.

Solution

For arrays >1024 elements, use the existing call_mlx_arg_sort function which implements multi-block merge sort from MLX. This
handles arbitrary array sizes efficiently.

Supported types for large arrays: BF16, F16, F32, U8, U32, I64

Notes:

• Small arrays (≤1024) still use bitonic sort for performance
• F64, I16, I32, F8E4M3 not yet supported for large arrays (will error with clear message)

Test plan

• asort_cpu - existing test passes
• asort_metal - existing test passes
• asort_big_cpu - tests 2000 elements
• asort_big_metal - now passes (was skipped)
• asort_vocabulary_cpu - tests 2048, 4096, 32000 elements
• asort_vocabulary_metal - tests vocabulary sizes used in LLMs

cargo test -p candle-core --test tensor_tests --features metal -- asort