The _generate_example method can be defined with any signature. This method is called only by the _split_generators method, and therefore, it is up to this method to correctly invoke _generate_example with the correct parameters.
Since we haven't overwritten the parent _split_generators method, we have to use the same signature required by the parent. Hence, we have the data_path and set_name parameters to use, in addition to all of the other information that is available in the PASCAL VOC 2007 documentation.
To goal of _generate_examples is to yield an example every time it is invoked (behaving like a standard Python iterator).
From the dataset structure, we know that, inside VOCdevkit/VOC2007/ImageSets/Segmentation/ ...
