There is a trick: rgrep -w spmm tools/autograd/ gives
tools/autograd/templates/python_torch_functions.cpp:  {"spmm", (PyCFunction)THPVariable_mm, METH_VARARGS | METH_KEYWORDS | METH_STATIC, NULL},

so it dispatches to mm.
You found that already, if you had not, I could offer a writeup of how PyTorch functions are generated 25.
Best regards
Thomas