This solved my problem.
def __next__(self):
        if(self._counter < len(self.index)):
            self.sample = self._data[self.index[self._counter] : self.index[self._counter] + self._chunk_size, :]
            self.label = self._labels[self.index[self._counter]]
            
            if self._mean_std_normalize:
                self.sample = (self.sample - 17041.3242) / (131.5367)
            
            print(self._counter)
            self._counter += 1
            return self.sample, self.label
        else:
             raise StopIteration

and moving self._counter = 0 to __iter__ method. Now, my __iter__ method is
def __iter__(self):
    self._counter = 0
    return self