It says you have two tensors whose size doesnt match. For example, you cant add two tensors if their sizes dont match.
What I usually do in such cases is spinkle print statements liberally just before the line in question, printing the .size() for the input tensors of the operaiton.
If you have a debugger, that might be faster. I never figured out how to use a python debugger yet  . Or perhaps I just find print sufficient/ok for me. Unclear…