code: def _get_statistics(self, x):
      dim2reduce = tuple(range(1, x.ndim - 1))
      if self.subtract_last:
          self.last = x[:, -1, :].unsqueeze(1)
      else:
          self.mean = torch.mean(
              x, dim=dim2reduce, keepdim=True
          ).detach()
      self.stdev = torch.sqrt(
          torch.var(x, dim=dim2reduce, keepdim=True, unbiased=False)
          + self.eps
      ).detach()
  
imports: import math
  import numpy
  from typing import Optional
  import torch
  import torch.nn
  import torch.nn.functional
  from ..common._base_windows import BaseWindows
  from ..losses.pytorch import MAE
label_desc: Use image node_id 4 for calling the RevIN obj's underlying member method code with example usage: obj._get_statistics(x) without return types