Add tests for C++ functional cosine_similarity and pairwise_distance, and clean up functional test code (#26559)
Summary:
This ensures that `F::cosine_similarity` and `F::pairwise_distance` can be used simply by including `torch/torch.h` and set `namespace F = torch::nn::functional`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26559
Differential Revision: D17507421
Pulled By: yf225
fbshipit-source-id: f895dde3634d5c8ca66ee036903e327e5cdab6b1
