Pretrained Model Weights Not Updating During DPO Training

Question

I'm trying to apply DPO to a pre-trained model. However, during the training process, the scores given by the pre-trained model and the fine-tuned model are identical, and the loss remains the same across all batches, leading me to believe the weights are not being updated. My training method is given below.

def train(model, optimizer, pref_set, dispref_set, epochs, beta, bs):
    model.train()
    #print(list(model.parameters())[0])
    #print(list(model.parameters())[0].grad)
    for epoch in range(epochs):
        cur_pref=[]
        cur_dispref=[]
        for i in range(len(pref_set)):
            cur_pref.append(pref_set[i])
            cur_dispref.append(dispref_set[i]) #collects preferred and dispreferred responses
            if (i+1) % bs == 0:
                make_fastas(cur_pref, cur_dispref) #sets up necessary files
                run_mpnn('model-DPO') #scores responses
                optimizer.zero_grad()
                b_ref, nb_ref, b_dpo, nb_dpo = collect_logps(cur_pref) #collects scores
                loss = calc_loss(b_dpo, nb_dpo, b_ref, nb_ref, beta) #computes DPO loss
                print(loss)
                loss.backward()
                optimizer.step()
                print(optimizer)
                torch.save({ #saves updated model for next round of scoring
                        'epoch': epoch+1,
                        'step': i,
                        'num_edges' : 48,
                        'noise_level': 0.2,
                        'model_state_dict': model.state_dict(),
                        'optimizer_state_dict': optimizer.state_dict(),
                        }, "../ProteinMPNN/vanilla_model_weights/model-DPO.pt")
                print(loss)
                cur_pref=[]
                cur_dispref=[]

In short, the scoring of my preferred and dispreferred responses must be done in a separate script, meaning I must save the updated model after each batch to be loaded for the following round of scoring. But as I mentioned, the model weights are not changing, and the scores returned by the reference and target models are always the same. Any help in resolving this issue would be greatly appreciated.

I've checked to make sure that the model parameters are initialized correctly, with requires_grad=True. They also have no gradient before training (list(model.parameters())[0].grad = None). I also checked to ensure that I'm not overwriting the updated model weights, or accidentally loading the vanilla weights during scoring. I double checked my loss function, and tried setting the loss and learning rates to arbitrarily high values to force the weights to update. However, no change in scoring occurred. The model parameter gradient after the backward call is still None, and I'm not sure why. As mentioned previously, all model parameters are initialized with requires_grad=True.