import tensorflow as tf
sess = tf.InteractiveSession()

i = tf.constant(0, dtype=tf.int32)
x = tf.constant(0, shape=[0], dtype=tf.int32)

i, x = tf.while_loop(lambda i, _: i < 10,
                     lambda i, x: (i + 1, tf.concat_v2([x, tf.expand_dims(i, 0)], 0)),
                     [i, x],
                     shape_variants=[i.get_shape(), tf.TensorShape([None])])

print(x.eval())

[0 1 2 3 4 5 6 7 8 9]