(fix): fix reset data idx.

[PanAndy]

self.idx = random.randint(0, len(self.dataset) - 1) may get idx=len(self.dataset), which will cause out of index error.

    def randint(self, a, b):
        """Return random integer in range [a, b], including both end points.
        """

        return self.randrange(a, b+1)