Sorry, I’m not sure what the problem is - it seems weird that PyTorch can’t convert a tensor into a tensor… My best guess is something strange is in act['a'], but it’s hard to say what exactly it could be.
Similar errors seem to happen here (where the dataset was loaded in incorrectly), and here (where there seem to have been compatibility issues), but both of those posts are with respect to tensorflow, not PyTorch, and they are only somewhat similar to your error.