[BE] Enforce missing override keyword (#104032)

This PR enables `-Winconsistent-missing-destructor-override` and `-Winconsistent-missing-override`
and fixes violations.

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### <samp>🤖 Generated by Copilot at 47e904e</samp>

This pull request updates the code of various classes and operators in the `caffe2` and `aten` subdirectories to use the `override` specifier instead of the `virtual` keyword for destructors and other virtual functions that override a base class function. This improves the code readability, quality, and consistency with C++ best practices. It also modifies the `./CMakeLists.txt` file to enable warnings for these specifiers, but disable errors.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/104032
Approved by: https://github.com/malfet

test/cpp/jit/test_backend_compiler_lib.cpp

@@ -68,8 +68,7 @@ class BackendWithCompiler : public PyTorchBackendInterface {
   // Constructor.
   // NOLINTNEXTLINE(modernize-use-equals-default)
   explicit BackendWithCompiler() {}
-  // NOLINTNEXTLINE(modernize-use-override)
-  virtual ~BackendWithCompiler() = default;
+  virtual ~BackendWithCompiler() override = default;
 
   bool is_available() override {
     return true;