Allow Users to Disable the Evaluation after the Last Training Iteration

Summary:
`Detectron2GoRunner` defaults to trigger an evaluation right after the last iteration in the `runner.do_train` method. This sometimes might be unnecessary, because there is a `runner.do_test` at the end of training anyways.

It could also lead to some side effects. For example, it would cause the training and test data loader present at the same time, which led to an OOM issue in our use case.

In this diff, we add an option `eval_after_train` in the `EvalHook` to allow users to disable the evaluation after the last training iteration.

Reviewed By: wat3rBro

Differential Revision: D34295685

fbshipit-source-id: 3612eb649bb50145346c56c072ae9ca91cb199f5

d2go/runner/default_runner.py

@@ -470,6 +470,7 @@ class Detectron2GoRunner(BaseRunner):
             hooks.EvalHook(
                 cfg.TEST.EVAL_PERIOD,
                 lambda: self.do_test(cfg, model, train_iter=trainer.iter),
+                eval_after_train=False,  # done by a separate do_test call in tools/train_net.py
             ),
             kmeans_anchors.compute_kmeans_anchors_hook(self, cfg),
             self._create_qat_hook(cfg) if cfg.QUANTIZATION.QAT.ENABLED else None,