add visualdl doc

docs/zh_CN/tutorials/getting_started.md

@@ -41,7 +41,8 @@ python -m paddle.distributed.launch \
     --selected_gpus="0,1,2,3" \
     tools/train.py \
         -c ./configs/ResNet/ResNet50_vd.yaml \
-        -o use_mix=1
+        -o use_mix=1 \
+	--vdl_dir=./scalar/
 
 ```
 
@@ -53,6 +54,13 @@ epoch:0    train    step:522    loss:1.6330    lr:0.100000    elapse:0.210
 
 也可以直接修改模型对应的配置文件更新配置。具体配置参数参考[配置文档](config.md)。
 
+训练期间可以通过VisualDL实时观察loss变化，启动命令如下：
+
+```bash
+visualdl --logdir ./scalar --host <host_IP> --port <port_num>
+
+```
+
 
 ### 2.2 模型微调
 

ppcls/utils/logger.py

@@ -82,6 +82,13 @@ def error(fmt, *args):
 
 
 def scaler(name, value, step, writer):
+    """
+    This function will draw a scalar curve generated by the visualdl.
+    Usage: Install visualdl: pip3 install visualdl==2.0.0b4
+           and then:
+           visualdl --logdir ./scalar --host 0.0.0.0 --port 8830 
+           to preview loss corve in real time.
+    """
     writer.add_scalar(name, value, step)