code: public SDVariable convertToConstant(@NonNull SDVariable variable) {
          convertToConstants(Collections.singletonList(variable));
          return variable;
      }
docs: Convert the specified variable to a constant. This is equivalent to "freezing" a variable so that it's value
  won't be changed by further training.<br>
  This can only be done for variables and placeholders, not ARRAY type variables (which are usually network activations).
  As a constant, this variable will no longer be modified by any subsequent training.<br>
  See also: {@link VariableType}
  
  @param variable Variable to convert to a constant
  @return The (now constant) SDVariable