code: def _npdf(x):
      return tf.math.exp(-0.5 * x**2) / _SQRT_2_PI
  
imports: from typing import Tuple
  import numpy
  import tensorflow.compat.v2
  from tf_quant_finance import types
  from tf_quant_finance.black_scholes import vanilla_prices
  from tf_quant_finance.math import gradient
  from tf_quant_finance.math import integration
  from tf_quant_finance.math.root_search import newton
label_desc: Use image node_id 15 for calling a global function with example usage: _npdf(x) and returns: unknown