title: C++ equivalent of numpy.expand_dims() and numpy.concatenate()
body: <p>As it mentioned in the title, I currently have some 2D images data read from OpenCV, I need to change the dimension to 4D. E.g., dimension <code>[320, 720]</code> to <code>[1, 320, 720, 1]</code>, and then make the entire data a single 4D matrix. </p>
  
  <p>In Python, I can just do <code>numpy.expand_dims()</code> for each of those images and then <code>numpy.concatenate()</code> them together. I'm wondering if there is some equivalent APIs that I could use in C++. I've found <code>expand_dims()</code> in <code>Tensorflow</code>, but it only works on tensors, and I haven't found anything for <code>concatenate()</code> yet. </p>
  
  <p>Libraries like <code>OpenCV</code>, <code>Tensorflow</code>, <code>Boost</code> are welcomed. But I want to keep things lighter, so it would be better if I can implement by myself (if not too complicated). Thank you in advance.</p>