text: import numpy as np
  from prml.nn.tensor.constant import Constant
  from prml.nn.tensor.tensor import Tensor
  from prml.nn.function import Function
  
  
  class Abs(Function):
  
      def forward(self, x):
          x = self._convert2tensor(x)
          self.x = x
          self.output = np.abs(x.value)
          if isinstance(x, Constant):
              return Constant(self.output)
          self.sign = np.sign(x.value)
          return Tensor(self.output, function=self)
  
      def backward(self, delta):
          dx = self.sign * delta
          self.x.backward(dx)
  
  
  def abs(x):
      """
      element-wise absolute function
      """
      return Abs().forward(x)
  