add `meshgrid` function

would be nice to have an equiv of tf.meshgrid in python tensorflow.

[silky]

  x = tf.linspace(-1, 1, w);
  y = tf.linspace(-1, 1, h);

  const xx = tf.matMul( tf.ones  ([h, 1]), x.reshape([1, w]) )
  const yy = tf.matMul( y.reshape([h, 1]), tf.ones  ([1, w]) );

vs. xx, yy = tf.meshgrid(x, y)

[rthadur]

@DCtheTall please go ahead , thank you.