As quoted from this blogpost on how PyTorch maps C code in Python

Now functions showing up in  torch  are more interesting - let’s take  torch.bilinear  - the function behind  torch.nn.functional.bilinear  - as an example. If you evaluate it (not call) on the IPython prompt, you’ll see  <function _VariableFunctions.bilinear>  and  print  (or  ? ) says it is built-in, i.e. defined in C. Sure enough, there is a  torch._C._VariableFunctions.bilinear . ( Note:  That  bilinear  is exported as  torch.bilinear  is somewhat  accidental . Do use the documented interfaces, here  torch.nn.functional.bilinear  whenever you can!)

Reference: https://lernapparat.de/selective-excursion-into-pytorch-internals/ 153