code: public SDVariable stack(String name, int axis, SDVariable... values) {
          validateSameType("stack", false, values);
          SDVariable ret = f().stack(values, axis);
          return updateVariableNameAndReference(ret, name);
      }
docs: Stack a set of N SDVariables of rank X into one rank X+1 variable.
  If inputs have shape [a,b,c] then output has shape:<br>
  axis = 0: [N,a,b,c]<br>
  axis = 1: [a,N,b,c]<br>
  axis = 2: [a,b,N,c]<br>
  axis = 3: [a,b,c,N]<br>
  
  @param name   Name of the output variable
  @param axis   Axis to stack on
  @param values Input variables to stack. Must have the same shape for all inputs
  @return Output variable
  @see #unstack(String[], SDVariable, int, int)