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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