Fix eval frequency

Former-commit-id: 1c6a9505c964fdd1bc7d55887725551afc94b956

train.py

@@ -89,7 +89,7 @@ def train_net(net,
 
                 pbar.update(imgs.shape[0])
                 global_step += 1
-                if global_step % (len(dataset) // (10 * batch_size)) == 0:
+                if global_step % (n_train // (10 * batch_size)) == 0:
                     for tag, value in net.named_parameters():
                         tag = tag.replace('.', '/')
                         writer.add_histogram('weights/' + tag, value.data.cpu().numpy(), global_step)