code: def __call__(self, t, x):
      self.nfe += 1
      if t < 0.5:
          return -0.5 * x
      else:
          return x**2
  
imports: from functools import partial
  import unittest
  import warnings
  import torch
  import torchdiffeq
  from problems import construct_problem, PROBLEMS, DTYPES, DEVICES, METHODS, ADAPTIVE_METHODS, FIXED_METHODS, SCIPY_METHODS
label_desc: Use image node_id 2 for calling the _JumpF obj's underlying member method code with example usage: obj.__call__(t, x) and returns: unknown, unknown