[webgpu] Convert i32 to u32 in uniforms (#26676)
In debug mode, `webgpu_context.cc:257 Run Uniform variable[5]
(head_size) data type mismatch in program
"SplitPackedQKVWithRotaryEmbeddingAndCopyKV", Expected: u32, Actual:
i32`. No issue in release mode.
Convert i32 to u32 to avoid this issue.
