Increase plot_labels() speed (#1736)

train.py

@@ -205,7 +205,7 @@ def train(hyp, opt, device, tb_writer=None, wandb=None):
             # cf = torch.bincount(c.long(), minlength=nc) + 1.  # frequency
             # model._initialize_biases(cf.to(device))
             if plots:
-                Thread(target=plot_labels, args=(labels, save_dir, loggers), daemon=True).start()
+                plot_labels(labels, save_dir, loggers)
                 if tb_writer:
                     tb_writer.add_histogram('classes', c, 0)