Update on "[ET-VK] Store unique ptr to StagingBuffer and SymInt in Value instead of inlined object, to reduce Value struct size from 80 to 32 bytes."

This diff aims to reduce the size of the `Value` struct from 80 bytes to 32 bytes by storing a unique pointer to `StagingBuffer` and `SymInt` instead of inlining the objects.

Differential Revision: [D66677759](https://our.internmc.facebook.com/intern/diff/D66677759/)

[ghstack-poisoned]

Author: trivedivivek

backends/vulkan/test/vulkan_compute_api_test.cpp

@@ -1087,8 +1087,8 @@ TEST_F(VulkanComputeAPITest, print_object_sizes) {
 
   // Current known size on 64 bit system: 1040 B
   EXPECT_TRUE(sizeof(vTensor) < 1200);
-  // Current known size on 64 bit system: 32 B
-  EXPECT_TRUE(sizeof(Value) < 40);
+  // Current known size on 64 bit system: 48 B
+  EXPECT_TRUE(sizeof(Value) < 56);
   // Current known size on 64 bit system: 120 B
   EXPECT_TRUE(sizeof(StagingBuffer) < 500);
   // Current known size on 64 bit system: 384 B