accumulate in fp32 for Reduce* (#19868)

Author: guschmue

js/web/lib/wasm/jsep/webgpu/ops/reduce-shared.ts

@@ -131,7 +131,7 @@ export const createReduceSharedProgramInfo =
       const workgroupSize = 32;
 
       const sharedMemorySnippet = `
-          var<workgroup> aBestValues : array<${output.type.storage}, ${workgroupSize}>;
+          var<workgroup> aBestValues : array<f32, ${workgroupSize}>;
        `;
 
       const getShaderSource = (shaderHelper: ShaderHelper) => `
@@ -145,10 +145,10 @@ export const createReduceSharedProgramInfo =
           let outputIndex = global_idx / ${workgroupSize};
           let offset = outputIndex * uniforms.reduceSize;
 
-          var bestValue = ${output.type.storage}(${reduceInitValues[reduceType]});
+          var bestValue = f32(${reduceInitValues[reduceType]});
           let Length = uniforms.reduceSize;
           for (var k = local_idx; k < Length; k = k + ${workgroupSize}) {
-           let candidate = ${output.type.storage}(${input.getByOffset('offset + k')});
+           let candidate = f32(${input.getByOffset('offset + k')});
            bestValue = ${reduceOps[reduceType]};
           }
           aBestValues[local_idx] = bestValue;
@@ -172,8 +172,8 @@ export const createReduceSharedProgramInfo =
           output.setByOffset(
               'outputIndex',
               `${
-                  reduceType === 'mean' ? `bestValue / ${output.type.storage}(uniforms.reduceSize)` :
-                                          `${reduceOutputValues[reduceType]}`}`)};
+                  reduceType === 'mean' ? `${output.type.storage}(bestValue / f32(uniforms.reduceSize))` :
+                                          `${output.type.storage}(${reduceOutputValues[reduceType]})`}`)};
          }
         }`;