text: Title: Performance issues in training_api/research/ (by P3)
  Body: Hello! I've found a performance issue in your program:
  
  - `tf.Session` being defined repeatedly leads to incremental overhead.
  
  You can make your program more efficient by fixing this bug. Here is [the Stack Overflow post](https://stackoverflow.com/questions/48051647/tensorflow-how-to-perform-image-categorisation-on-multiple-images) to support it.
  
  Below is detailed description about **tf.Session being defined repeatedly**:
  
  - in object_detection/eval_util.py: `sess = tf.Session(master, graph=tf.get_default_graph())`[(line 273)](https://github.com/BMW-InnovationLab/BMW-TensorFlow-Training-GUI/blob/ecf941242e3c7380d2e6060652d209509bc9f224/training_api/research/object_detection/eval_util.py#L273) is defined in the function `_run_checkpoint_once`[(line 211)](https://github.com/BMW-InnovationLab/BMW-TensorFlow-Training-GUI/blob/ecf941242e3c7380d2e6060652d209509bc9f224/training_api/research/object_detection/eval_util.py#L211) which is repeatedly called in the loop `while True:`[(line 431)](https://github.com/BMW-InnovationLab/BMW-TensorFlow-Training-GUI/blob/ecf941242e3c7380d2e6060652d209509bc9f224/training_api/research/object_detection/eval_util.py#L431).
  - in slim/datasets/download_and_convert_cifar10.py: `with tf.Session('') as sess:`[(line 91)](https://github.com/BMW-InnovationLab/BMW-TensorFlow-Training-GUI/blob/ecf941242e3c7380d2e6060652d209509bc9f224/training_api/research/slim/datasets/download_and_convert_cifar10.py#L91) is defined in the function `_add_to_tfrecord`[(line 64)](https://github.com/BMW-InnovationLab/BMW-TensorFlow-Training-GUI/blob/ecf941242e3c7380d2e6060652d209509bc9f224/training_api/research/slim/datasets/download_and_convert_cifar10.py#L64) which is repeatedly called in the loop `for i in range(_NUM_TRAIN_FILES):`[(line 184)](https://github.com/BMW-InnovationLab/BMW-TensorFlow-Training-GUI/blob/ecf941242e3c7380d2e6060652d209509bc9f224/training_api/research/slim/datasets/download_and_convert_cifar10.py#L184).
  
  `tf.Session` being defined repeatedly could lead to incremental overhead. If you define `tf.Session` out of the loop and pass `tf.Session` as a parameter to the loop, your program would be much more efficient.
  
  Looking forward to your reply. Btw, I am very glad to create a PR to fix it if you are too busy.