Hey,
All the C++ / CUDA implementations are present in ATen, and can be found in https://github.com/pytorch/pytorch/tree/master/aten 2
For your questions in particular, cadd can be seen in here 4 for CUDA, and here 3 for the CPU implementation.
But many operations are moving straight to ATen, which gives more flexibility and nicer interfaces, see for example advanced indexing in ATen 1