[ET-VK] Adding reserve and append functions to SpecVarList

Pull Request resolved: #8633

This diff adds two new functions to the SpecVarList class in the Vulkan runtime library. The first function, reserve, allows the user to reserve a certain amount of space in the SpecVarList before adding any elements. The second function, append, allows the user to add a single SpecVar to the SpecVarList. These functions are useful for optimizing memory usage and improving performance in the Vulkan runtime.
ghstack-source-id: 268172659
@exported-using-ghexport

Differential Revision: [D70021782](https://our.internmc.facebook.com/intern/diff/D70021782/)

Author: trivedivivek

backends/vulkan/runtime/vk_api/Pipeline.cpp

@@ -174,6 +174,14 @@ void SpecVarList::append(const SpecVarList& other) {
   vars.insert(vars.end(), other.vars.begin(), other.vars.end());
 }
 
+void SpecVarList::reserve(const size_t size) {
+  vars.reserve(size);
+}
+
+void SpecVarList::append(const SpecVar& other) {
+  vars.push_back(other);
+}
+
 std::vector<VkSpecializationMapEntry> SpecVarList::generate_map_entries()
     const {
   std::vector<VkSpecializationMapEntry> map_entries;

backends/vulkan/runtime/vk_api/Pipeline.h

@@ -82,6 +82,10 @@ class SpecVarList final {
 
   void append(const SpecVarList& other);
 
+  void reserve(const size_t size);
+
+  void append(const SpecVar& other);
+
   std::vector<VkSpecializationMapEntry> generate_map_entries() const;
 
   friend bool operator==(const SpecVarList& lhs, const SpecVarList& rhs);