fix typo for faster_rcnn_50_2x.yml (#2515)

* Fix type for faster_rcnn_50_2x.
* Make sure that type of cpu_num is int.

PaddleCV/object_detection/configs/faster_rcnn_r50_2x.yml

@@ -43,7 +43,7 @@ RPNHead:
   train_proposal:
     min_size: 0.0
     nms_thresh: 0.7
-    pre_nums_top_n: 12000
+    pre_nms_top_n: 12000
     post_nms_top_n: 2000
   test_proposal:
     min_size: 0.0

PaddleCV/object_detection/tools/eval.py

@@ -51,7 +51,7 @@ def main():
     if cfg['use_gpu']:
         devices_num = fluid.core.get_cuda_device_count()
     else:
-        devices_num = os.environ.get('CPU_NUM', multiprocessing.cpu_count())
+        devices_num = int(os.environ.get('CPU_NUM', multiprocessing.cpu_count()))
 
     if 'eval_feed' not in cfg:
         eval_feed = create(main_arch + 'EvalFeed')

PaddleCV/object_detection/tools/train.py

@@ -53,7 +53,7 @@ def main():
     if cfg['use_gpu']:
         devices_num = fluid.core.get_cuda_device_count()
     else:
-        devices_num = os.environ.get('CPU_NUM', multiprocessing.cpu_count())
+        devices_num = int(os.environ.get('CPU_NUM', multiprocessing.cpu_count()))
 
     if 'train_feed' not in cfg:
         train_feed = create(type(main_arch).__name__ + 'TrainFeed')