Mayank_Jha:

I have conctenated the output from already trained NN (called "model " in the code below),
with another data coming in from data loader,


That’s not the case in the posted code snippet.
You are directly passing the output (output_sys) of the trained model to the criterion and use this loss to calculate the gradients.
In the code snippet the output of modelFCNN, which is supposed to be trained, is concatenated with the data from the DataLoader.
Is this an error in the code or in the description?