update loss_select.md and model_builder.py (#67)

* update model_builder.py

* update loss_select.md

docs/loss_select.md

@@ -55,13 +55,13 @@ python pdseg/check.py --cfg ./configs/deepglobe_road_extraction.yaml
 
 * 训练
 ```shell
-python pdseg/train.py --cfg ./configs/deepglobe_road_extraction.yaml --use_gpu SOLVER.LOSS ['dice_loss','bce_loss']
+python pdseg/train.py --cfg ./configs/deepglobe_road_extraction.yaml --use_gpu SOLVER.LOSS "['dice_loss','bce_loss']"
 
 ```
 
 * 评估
 ```
-python pdseg/eval.py --cfg ./configs/deepglobe_road_extraction.yaml --use_gpu SOLVER.LOSS ['dice_loss','bce_loss']
+python pdseg/eval.py --cfg ./configs/deepglobe_road_extraction.yaml --use_gpu SOLVER.LOSS "['dice_loss','bce_loss']"
 
 ```
 

pdseg/models/model_builder.py

@@ -158,6 +158,8 @@ def build_model(main_prog, start_prog, phase=ModelPhase.TRAIN):
             model_func = get_func("modeling." + model_name)
 
             loss_type = cfg.SOLVER.LOSS
+            if not isinstance(loss_type, list):
+                loss_type = list(loss_type)
 
             if class_num > 2 and (("dice_loss" in loss_type) or ("bce_loss" in loss_type)):
                 raise Exception("dice loss and bce loss is only applicable to binary classfication")