Fix missing class in cpp tensor documentation (#54488)
Summary:
The given example in the documentation does not compile due to the missing `torch::`. It is correct in the tutorial about [writing a custom extension ](https://pytorch.org/tutorials/advanced/cpp_extension.html#writing-a-mixed-c-cuda-extension)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54488
Reviewed By: bdhirsh
Differential Revision: D27267000
Pulled By: glaringlee
fbshipit-source-id: 86a46d656c1a4fa4098287a6a43a38d1ef80171e
