[webgpu] Fix access mode for storage address space (#6607)

* [webgpu] Fix access mode for storage address space

BUG
Due to WGSL Spec(https://www.w3.org/TR/WGSL/#address-space), storage
address space only supported read_write and read access modes.

* Change module-scope 'let' -> 'const'

Author: haoyunfeix

tfjs-backend-webgpu/src/matmul_packed_webgpu.ts

@@ -177,7 +177,7 @@ export function makeMatMulVectorSource(
       workGroupSize[1] === 1 && workGroupSize[2] === 1,
       () => `A linear work group size is required. But got ${workGroupSize}.`);
   return `
-    let TileSize = ${workGroupSize[0] * 4};
+    const TileSize = ${workGroupSize[0] * 4};
     var<workgroup> mm_Asub : array<vec4<f32>, ${workGroupSize[0]}>;
 
     ${getMainHeaderString()}

tfjs-backend-webgpu/src/webgpu_program.ts

@@ -167,7 +167,7 @@ function makeShader(
           outShapeStrides : vec2<i32>,
         };
 
-        @group(0) @binding(0) var<storage, write> result: array<${
+        @group(0) @binding(0) var<storage, read_write> result: array<${
         mapToWgslTypes(outputData.dtype, program.isVec4)}>;
         @group(0) @binding(2) var<uniform> uniforms: Uniform;
       `);
@@ -238,7 +238,7 @@ function makeShader(
     `);
   } else {
     prefixSnippets.push(`
-      @group(0) @binding(0) var<storage, write> result: array<${
+      @group(0) @binding(0) var<storage, read_write> result: array<${
         mapToWgslTypes(outputData.dtype, program.isVec4)}>;
     `);
   }