bench: fix 4-bit byte accounting in benchmark metrics

[Pritiks23]

u4/s4 benchmark metrics are now accounted as 0.5 bytes per element, and related benchmark utility tests were updated accordingly.

[Pritiks23]

Updated the benchmark metric logic so 4-bit matrix types are counted correctly in memory calculations.

What changed:

1. In the benchmark metrics code, u4 and s4 are now treated as 0.5 bytes per element instead of 1 byte per element.
2. The type-size helper now returns a floating-point value so it can represent half-byte element sizes.
3. Benchmark utility tests were updated to match the new behavior (u4/s4 expected value changed from 1.0 to 0.5).

Why this change was needed:

1. The previous logic overestimated memory traffic for 4-bit data by 2x.
2. That made bandwidth and matrix-size metrics inaccurate for low-bit GEMM benchmarks.
3. With this fix, reported benchmark metrics better reflect actual packed 4-bit storage, so performance analysis is more trustworthy.