code: def _cdf(x):
      return (tf.math.erf(x / _SQRT_2) + 1) / 2
  
file_path: tf-quant-finance/tf_quant_finance/black_scholes/implied_vol_newton_root.py
imports: import numpy
  import tensorflow.compat.v2
  from tf_quant_finance.black_scholes import implied_vol_approximation
  from tf_quant_finance.black_scholes import implied_vol_utils
  from tf_quant_finance.math.root_search import newton
label_desc: Use image node_id 1 for calling a global function with example usage: _cdf(x) and returns: unknown