docs: public void execBackwards(Map<String,INDArray> placeholders, List<String> variableGradNamesList){
          if (getFunction("grad") == null) {
              createGradFunction();
          }
  
          log.trace("About to execute backward function");
  
          //Edge case: if no variables, no variable gradients to calculate...
          if(variableGradNamesList.isEmpty()){
              log.warn("Skipping gradient calculation (backward pass) - no variables to be calculated (variableGradNamesList is empty)");
              return;
          }
  
          sameDiffFunctionInstances.get("grad").exec(placeholders, variableGradNamesList);
      }