'given index, find out sub-db and sub-index
Parameters
index : int
index of a specific image
Returns
a tuple (sub-db, sub-index)'

def _locate_index(self, index):

    assert ((index >= 0) and (index < self.num_images)), 'index    out    of    range'
    pos = self.image_set_index[index]
    for (k, v) in enumerate(self.imdbs):
        if (pos >= v.num_images):
            pos -= v.num_images
        else:
            return (k, pos)
