[ET-VK][ez] Make get_tensor() API protected (#13168)

## Changes

As title; make the `get_tensor()` API protected.

## Motivation

See the below diff/PR in the stack. The goal is to encourage operator authors to go through the `ComputeGraph` to access/modify tensors so that the activity can be tracked.

Differential Revision: [D79564596](https://our.internmc.facebook.com/intern/diff/D79564596/)

Author: pytorchbot

backends/vulkan/runtime/graph/ComputeGraph.h

@@ -248,7 +248,16 @@ class ComputeGraph final {
     return values_.at(idx).is##type_name();                                \
   }
 
-  GET_AND_CHECK_VAL_AS_PTR_TYPE_FNS(vTensorPtr, tensor, Tensor)
+ protected:
+  inline vTensorPtr get_tensor(const ValueRef idx) {
+    return vTensorPtr(this, idx);
+  }
+
+ public:
+  inline bool val_is_tensor(const ValueRef idx) const {
+    return values_.at(idx).isTensor();
+  }
+
   GET_AND_CHECK_VAL_AS_PTR_TYPE_FNS(TensorRefPtr, tref, TensorRef)
   GET_AND_CHECK_VAL_AS_PTR_TYPE_FNS(StagingPtr, staging, Staging)
   GET_AND_CHECK_VAL_AS_PTR_TYPE_FNS(IntListPtr, int_list, IntList)
@@ -970,6 +979,8 @@ class ComputeGraph final {
   friend class SymIntPtr;
 
   friend struct TmpTensor;
+  friend struct SharedObject;
+  friend class BlitNode;
 };
 
 template <typename T>