Don't see any transfers on NVLINK with NCCL all_sum test

Question

With the following code (uses tensorflow.contrib.nccl.all_sum), I expected to see bytes being transferred over NVLINK. In reality, I don't.

from tensorflow.contrib.nccl import all_sum 

with tf.device('/gpu:0'):
        a = tf.get_variable(
            "a", initializer=tf.constant(1.0, shape=(args.dim, args.dim)))

with tf.device('/gpu:1'):
        b = tf.get_variable(
            "b", initializer=tf.constant(2.0, shape=(args.dim, args.dim)))

with tf.device('/gpu:0'):
        summed_node = all_sum([a, b])
         sess = tf.Session(config=tf.ConfigProto(allow_soft_placement=True,
                                            log_device_placement=True))

init = tf.global_variables_initializer() 
sess.run(init)

with tf.device('/gpu:0'):
        summed = sess.run(summed_node)

My machine is an AWS instance of p3.8xlarge. My understanding is, this configuration supports NVLINK.

The execution is fine but when I use nvidia-smi nvlink -g 0 -i 0 the link Tx/Rx counts are zero.