fix openai gpt example and updating readme

examples/run_openai_gpt.py

@@ -210,6 +210,7 @@ def main():
                 loss = args.lm_coef * losses[0] + losses[1]
                 loss.backward()
                 optimizer.step()
+                optimizer.zero_grad()
                 tr_loss += loss.item()
                 exp_average_loss = loss.item() if exp_average_loss is None else 0.7*exp_average_loss+0.3*loss.item()
                 nb_tr_steps += 1