Add metal count equal op

[gatbontonpc]

Add metal count equal op

This PR extends the CPU implementations of count_equal to Metal.

The current implementation uses a single thread group, but supports multiple if anything changes. This currently matches the CPU / Cuda implementation in which only takes int32 for src0 and src1. This kernel uses the atomic_fetch_add_explicit, which only supports returning an int32 adds similar to Cuda. This limits the size of the buffers we can take in to 2^31 - 1.

The docs have been updated.

codex generated summary:

Summary

This PR introduces a Metal implementation for COUNT_EQUAL on int32 tensors that uses SIMD-group reduction to efficiently compute per-threadgroup partial counts and accumulate the result into the destination buffer using atomic operations.

The change improves parallel efficiency over a naïve per-element atomic approach by:

• Performing the equality comparison per thread
• Reducing results within a SIMD group via simd_sum
• Emitting a single atomic update per SIMD group

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Key Changes

• Added a templated Metal kernel kernel_count_equal<int32_t>
• Uses shared memory (shmem_i32) and SIMD intrinsics (simd_sum) to aggregate counts
• Emits a single atomic_fetch_add_explicit per SIMD group
• Registers kernel under the exported symbol:

  kernel_count_equal_i32

[ggerganov]

This does not work, you need to call a separate kernel that fills the buffer with zeros

[gatbontonpc]

Added a new kernel to memset a buffer to a value. Similar to fill but simpler pipeline and only takes the buffer and value.