[WebGPU] fix test failure in Reduce operators on macOS ARM64 (#24108)
### Description
fix test failure in Reduce operators on macOS ARM64
```
[E:onnxruntime:ReduceL1, sequential_executor.cc:572 ExecuteKernel] Non-zero status code returned while running ReduceL1 node. Name:'node1' Status Message: webgpu_context.cc:259 Run Uniform variable[0] (output_size) data type mismatch in program "ReduceL1", Expected: u32, Actual: i32
```