Found a solution…
Prior to the line ans0 = mu_law_decode(np.concatenate(listofpred0)), I added a for loop that went through all elements in listofpred0 and assigned to CPU one by one. Here is the updated code:
def test(epoch):  # testing data
    model.eval()
    start_time = time.time()
    with torch.no_grad():
        for iloader, xtrain in loadtest:
            iloader=iloader.item()
            print(iloader)
            listofpred0 = []
            for ind in range(0, xtrain.shape[-1] - sampleSize, sampleSize):
                output = model(xtrain[:, :,ind:ind + sampleSize].to(device))
                listofpred0.append(output.reshape(-1))
            # New For Loop
            for i in range(0,len(listofpred0)):
                listofpred0[i] = listofpred0[i].cpu()
            ans0 = mu_law_decode(np.concatenate(listofpred0))
            if not os.path.exists('vsCorpus/'): os.makedirs('vsCorpus/')
            sf.write(savemusic.format(iloader), ans0, sample_rate)
            print('test stored done', np.round(time.time() - start_time))