fix one build warning in MSVC (#21983)

### Description

Fix one MSVC warning member not initialized


```
Warning	C26495	Variable 'onnxruntime::ITuningContext::allocators_' is uninitialized. Always initialize a member variable (type.6).  C:\code\onnxruntime\onnxruntime\core\framework\tuning_context.h	22		
```

onnxruntime/core/framework/tuning_context.h

@@ -19,7 +19,7 @@ class TuningResultsValidator;
 
 class ITuningContext {
  public:
-  explicit ITuningContext(IExecutionProvider* ep) : ep_(ep) {}
+  explicit ITuningContext(IExecutionProvider* ep) : ep_(ep), allocators_(nullptr) {}
   virtual ~ITuningContext() = default;
 
   virtual void EnableTunableOp() = 0;