Mark overriden Tensor method `override` (#47198)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47198
Fixes:
```xplat/caffe2/aten/src/ATen/native/xnnpack/OpContext.h:77:10: error: 'run' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
Tensor run(const Tensor& input);```
Test Plan: CI tests
Reviewed By: kimishpatel
Differential Revision: D24678573
fbshipit-source-id: 244769cc36d3c1126973a67441aa2d06d2b83b9c
