code: def _to_unconstrained(x, lb, ub):
      """Scale and apply inverse-sigmoid."""
      x = _scale(x, lb, ub)
      return -tf.math.log((1.0 - x) / x)
  
imports: from typing import Callable, Tuple
  import tensorflow.compat.v2
  import tf_quant_finance
  from tf_quant_finance import types
  from tf_quant_finance import utils
  from tf_quant_finance.math import make_val_and_grad_fn
  from tf_quant_finance.math import optimizer
  from tf_quant_finance.models.heston import approximations
label_desc: Use image node_id 4 for calling a global function with example usage: _to_unconstrained(x, lb, ub) and returns: unknown