[FasterRCNN] clear CUDA_VISIBLE_DEVICES in data may cause error (#494)

examples/FasterRCNN/utils/box_ops.py

@@ -2,7 +2,6 @@
 # -*- coding: utf-8 -*-
 # File: box_ops.py
 
-import os
 import tensorflow as tf
 from tensorpack.tfutils.scope_utils import under_name_scope
 from tensorpack.tfutils import get_default_sess_config
@@ -75,12 +74,12 @@ def get_iou_callable():
     """
     Get a pairwise box iou callable.
     """
-    os.environ['CUDA_VISIBLE_DEVICES'] = ''  # we don't want the dataflow process to touch CUDA
+    # We don't want the dataflow process to touch CUDA
+    # Data needs tensorflow. As a result, the training cannot run on GPUs with
+    # EXCLUSIVE_PROCESS mode, unless you disable multiprocessing prefetch.
     with tf.Graph().as_default(), tf.device('/cpu:0'):
         A = tf.placeholder(tf.float32, shape=[None, 4])
         B = tf.placeholder(tf.float32, shape=[None, 4])
         iou = pairwise_iou(A, B)
-        tf.logging.set_verbosity(tf.logging.FATAL)  # TF will warn about GPU not found
         sess = tf.Session(config=get_default_sess_config())
-        tf.logging.set_verbosity(tf.logging.INFO)
         return sess.make_callable(iou, [A, B])