code: def _default_vol_data(dimension, dtype):
      """Unit volatility and corresponding covariance functions."""
  
      def unit_vol(time):
          shape = tf.concat(
              [tf.shape(time), [dimension, dimension]], axis=0
          )
          out_dtype = tf.convert_to_tensor(time, dtype=dtype).dtype
          return tf.ones(shape, dtype=out_dtype)
  
      def covar(start_time, end_time):
          dt = tf.convert_to_tensor(
              end_time - start_time, dtype=dtype, name="dt"
          )
          return outer_multiply(dt, tf.eye(dimension, dtype=dt.dtype))
  
      return unit_vol, covar
  
file_path: tf-quant-finance/tf_quant_finance/models/legacy/brownian_motion_utils.py
imports: import tensorflow.compat.v2
  from tf_quant_finance.math import gradient
label_desc: Use image node_id 8 for calling a global function with example usage: _default_vol_data(dimension, dtype) and returns: unit_vol,  covar, tf, outer_multiply