[Enhancement] Update user api and docs with tools/xxx (PaddlePaddle#2603)

Author: juncaipeng

configs/pp_liteseg/README.md

@@ -54,7 +54,7 @@ export model=pp_liteseg_stdc1_cityscapes_1024x512_scale0.5_160k     # test resol
 # export model=pp_liteseg_stdc2_cityscapes_1024x512_scale1.0_160k
 # export model=pp_liteseg_stdc1_camvid_960x720_10k
 # export model=pp_liteseg_stdc2_camvid_960x720_10k
-python -m paddle.distributed.launch train.py \
+python -m paddle.distributed.launch tools/train.py \
     --config configs/pp_liteseg/${model}.yml \
     --save_dir output/${model} \
     --save_interval 1000 \
@@ -77,7 +77,7 @@ Refer to [doc](../../docs/evaluation/evaluate/evaluate.md) for the detailed usag
 export CUDA_VISIBLE_DEVICES=0
 export model=pp_liteseg_stdc1_cityscapes_1024x512_scale0.5_160k
 # export other model
-python val.py \
+python tools/val.py \
     --config configs/pp_liteseg/${model}.yml \
     --model_path output/${model}/best_model/model.pdparams \
     --num_workers 3

configs/pssl/README.md

@@ -36,7 +36,6 @@ Make sure that the datasets have structures as follows:
 
 ```
 PaddleSeg
-│   train.py
 │   ...  
 │
 └───data
@@ -72,7 +71,7 @@ Having installed PaddlePaddle and PaddleSeg and prepared datasets (ImageNet and
 export CUDA_VISIBLE_DEVICES=0,1,2,3,4,5,6,7
 
 path_save="work_dirs_stdc2_pssl"
-python -m paddle.distributed.launch --log_dir $path_save train.py \
+python -m paddle.distributed.launch --log_dir $path_save tools/train.py \
        --config configs/pssl/stdc2_seg_pssl.yml \
        --log_iters 200 \
        --num_workers 12 \
@@ -87,7 +86,7 @@ python -m paddle.distributed.launch --log_dir $path_save train.py \
 export CUDA_VISIBLE_DEVICES=0,1,2,3,4,5,6,7
 
 path_save="work_dirs_pp_liteseg_stdc2_pssl"
-python -m paddle.distributed.launch --log_dir $path_save train.py \
+python -m paddle.distributed.launch --log_dir $path_save tools/train.py \
        --config configs/pssl/pp_liteseg_stdc2_pssl.yml \
        --log_iters 100 \
        --num_workers 12 \