Fix C++ at::Tensor docs generation (#34467)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/25845.
**Test Plan:**
Check `pytorch_cpp_doc_push` CI job, and see if there is `classat_1_1_tensor` generated (similar to `structat_1_1native_1_1_convolution_descriptor`).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34467
Differential Revision: D20338190
Pulled By: yf225
fbshipit-source-id: 52dc05af5e0d742e740de5576d0d2b3e17ef28dd
