Add Virtual Destructor to FlatIndex Class (#4381)

Summary:
Pull Request resolved: #4381

This change introduces a virtual destructor to the FlatIndex class to ensure proper cleanup of derived class objects when they are deleted through a base class pointer. This modification addresses the compiler warning about deleting objects with virtual functions but non-virtual destructors, thereby preventing potential resource leaks and undefined behavior.

Original Errors:
```
Remote action, reproduce with: `frecli cas download-action 5c1f300e23c7f3a3c0ac1e5644473bfc3940abade7d8c1496bf2adcb5b1c8181:145`
Stdout: <empty>
Stderr:
fbcode/third-party-buck/platform010/build/libgcc/include/c++/trunk/bits/unique_ptr.h:85:1: error: delete called on non-final 'faiss::gpu::FlatIndex' that has virtual functions but non-virtual destructor [-Werror,-Wdelete-non-abstract-non-virtual-dtor]
   85 | delete __ptr;
      | ^
fbcode/third-party-buck/platform010/build/libgcc/include/c++/trunk/bits/unique_ptr.h:361:1: note: in instantiation of member function 'std::default_delete<faiss::gpu::FlatIndex>::operator()' requested here
  361 | get_deleter()(std::move(__ptr)); }
      | ^
fbcode/faiss/gpu/GpuIndexFlat.cu:28:15: note: in instantiation of member function 'std::unique_ptr<faiss::gpu::FlatIndex>::~unique_ptr' requested here
   28 | GpuIndexFlat::GpuIndexFlat(GpuResourcesProvider *
      |               ^
1 error generated.
```

Reviewed By: zhuhan0

Differential Revision: D76303668

fbshipit-source-id: 27965d6dc4fd03366d599be8a3760be1df668d5b

Author: wenxin0319

faiss/gpu/impl/FlatIndex.cuh

@@ -97,6 +97,9 @@ class FlatIndex {
     /// Free all storage
     void reset();
 
+    // Add a virtual destructor to avoid warnings
+    virtual ~FlatIndex() {}
+
    protected:
     /// Collection of GPU resources that we use
     GpuResources* resources_;