Update early stopping tutorial to use TensorboardMetric instead of Te…

…nsorboardCurveMetric (#2635)

Summary:
Pull Request resolved: #2635

D57922964 reaped the old TB integration, updating the tutorial to use the new TB integration. This is more or less identical to what we did for pytorch/tutorials here pytorch/tutorials#2985

Reviewed By: Balandat

Differential Revision: D60767153

fbshipit-source-id: fe2ca5db693ee51cad35ca79b076c283dc76da2f