Update proposal_target_layer_tf.py

[wangershi]

type of "bg_rois_per_this_image" may be float, in this case it will raise a error like "TypeError: 'float' object cannot be interpreted as an integer"

[chrenlily]

你好!我想问一下我在运行的时候出现TypeError: 'float' object cannot be interpreted as an integer",将代码更改后出现了"段错误 (核心已转储)",是我代码修改不对吗?