docs: public SDVariable grad(String varName) {
          if (!sameDiffFunctionInstances.containsKey("grad")) {
              throw new IllegalStateException("Unable to obtain gradient. Please run execBackwards() first.");
          }
  
          SameDiff grad = getFunction("grad");
          SDVariable var = grad.getVariable(varName);
          return getFunction("grad").getGradForVariable(var.getVarName());
      }