docstr: Alter the Contrast of an image with a value randomly selected
  from the list of given values with given probabilities
  
  Arguments
  ---------
  values : list of floats
      contrast values to sampled from
  p : list of floats - same length as `values`
      if None, values will be sampled uniformly.
      Must sum to 1.
code: def __init__(self, values, p=None):
      """
      Alter the Contrast of an image with a value randomly selected
      from the list of given values with given probabilities
  
      Arguments
      ---------
      values : list of floats
          contrast values to sampled from
      p : list of floats - same length as `values`
          if None, values will be sampled uniformly.
          Must sum to 1.
  
      """
      self.values = values
      self.p = p
  
imports: import random
  import torch
  from torchvision.transforms.functional import to_tensor
  import numpy
  from ..utils import th_random_choice
label_desc: Use image node_id 1 to create a new RandomChoiceContrast object  with example: obj = RandomChoiceContrast(values, p)