[Metal] Avoid regex in custom kernel name generation

[aleroot]

Proposed changes

Templated custom Metal kernels generate a specialized kernel name on every invocation. This previously constructed a std::regex and called std::regex_replace to convert the template definition into a valid kernel-name suffix.

This cost becomes measurable in workloads that repeatedly invoke templated custom kernels. In an Instruments capture of Qwen 3.5 gated-delta decoding, regex processing accounted for approximately 3.5% of the generation worker's sampled CPU cycles.

This change replaces the regex with a single linear pass over the template definition while preserving the generated kernel names byte-for-byte. It also removes the unused <regex> dependency.

Performance

An isolated C++20 -O3 benchmark using a representative template definition:

<bfloat16_t, float, 128, 64, 8, 8>

Results over 20,000 iterations:

• Previous regex implementation: approximately 6.86 µs per call
• New linear implementation: approximately 0.077 µs per call
• Isolated speedup: approximately 88.6×

This benchmark measures only template-name transformation and does not imply an equivalent end-to-end inference speedup.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (not needed: this is an internal performance change with no API or behavior change)