[WebNN EP] Add cache for MLContexts in the WebNNBackend (microsof…

…t#22510)

### Description
This change adds a cache of `MLContext`s keyed by their options to the
`WebNNBackend`. This makes is so that multiple `InferenceSession`s
create with the same options will share the same context.

### Motivation and Context
Since `MLTensor`s are tied `MLContext`s, developer can't easily share
tensors between `InferenceSession` (outside of manually an `MLContext`
and specifying the `context` options). This leads strange behaviors such
as,
```js
const sessionsA = ort.InferenceSession.create(urlA, {
  executionProviders: ["webnn"],
  preferredOutputLocation: "ml-buffer",
});
const sessionsB = ort.InferenceSession.create(urlB, {
  executionProviders: ["webnn"],
});
const temp = await sessionA.run({/* arguments */});
const result = await sessionB.run({"input":temp["output"]}); // ERROR: Failed to execute 'dispatch' on 'MLContext': Invalid inputs: The context of MLGraph doesn't match the context of the MLTensor with name "input".
```
We encountered this behavior when updating the transformers.js version
in the developer preview demos. microsoft/webnn-developer-preview#46

js/web/lib/wasm/wasm-types.ts

@@ -232,23 +232,6 @@ export declare namespace JSEP {
      * @returns
      */
     jsepCreateMLContext(optionsOrGpuDevice?: MLContextOptions | GPUDevice): Promise<MLContext>;
-
-    /**
-     * [exported from pre-jsep.js] Register a WebNN Constant operand from external data.
-     * @param externalFilePath - specify the external file path.
-     * @param dataOffset - specify the external data offset.
-     * @param dataLength - specify the external data length.
-     * @param builder - specify the MLGraphBuilder used for constructing the Constant.
-     * @param desc - specify the MLOperandDescriptor of the Constant.
-     * @returns the WebNN Constant operand for the specified external data.
-     */
-    jsepRegisterMLConstant(
-      externalFilePath: string,
-      dataOffset: number,
-      dataLength: number,
-      builder: MLGraphBuilder,
-      desc: MLOperandDescriptor,
-    ): MLOperand;
   }
 }
 

onnxruntime/wasm/pre-jsep.js

@@ -237,11 +237,13 @@ Module['jsepInit'] = (name, params) => {
     }
     Module['jsepRegisterMLTensor'] = (tensor, dataType, shape) => {
       return backend['registerMLTensor'](tensor, dataType, shape);
-    }
-
+    };
+    Module['jsepCreateMLContext'] = (optionsOrGpuDevice) => {
+      return backend['createMLContext'](optionsOrGpuDevice);
+    };
     Module.jsepRegisterMLConstant = (externalFilePath, dataOffset, dataLength, builder, desc) => {
       return backend['registerMLConstant'](
           externalFilePath, dataOffset, dataLength, builder, desc, Module.MountedFiles);
-    }
+    };
   }
 };