code: def _pdf(x):
      return tf.math.exp(-0.5 * x**2) / _SQRT_2_PI
  
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 2 for calling a global function with example usage: _pdf(x) and returns: unknown