Reference is from this github issue.
When the error is RuntimeError: Expected object of scalar type Float but got scalar type Double for argument #4 'mat1', you would need to use the .float() function since it says Expected object of scalar type Float.
Therefore, the solution is changing y_pred = model(X_trainTensor) to y_pred = model(X_trainTensor.float()).
Likewise, when you get another error for loss = loss_fn(y_pred, y_trainTensor), you need y_trainTensor.long() since the error message says Expected object of scalar type Long.
You could also do model.double(), as suggested by @Paddy
.