In [12]:
biases = {
  'b1' : tf.Variable(tf.zeros([n_hidden_unit_1])), 
  'b2' : tf.Variable(tf.zeros([n_output_unit])) 
}

# logging our biases
tf.summary.histogram("Biases_1__", biases["b1"])
tf.summary.histogram("Biases_2__", biases["b2"])
Out[12]:
<tf.Tensor 'Biases_2__:0' shape=() dtype=string>