docs: def tf_compilation_index(self):
          """
          Takes out index from the parameter's tensor name. E.g. parameter tensor name is 
          GPR-0000/kern/lengthscales, the method for that parameter will return '0000' index.
          """
          if self.parameter_tensor is None:
              return None
          name = self.parameter_tensor.name
          return name.split('-', 1)[-1].split('/')[0]