GAN task

Summary:
This diff introduces the D2 (https://github.com/facebookresearch/d2go/commit/9d9f438b191634dc38d16f3973e490909b7f67dd)Go GANs Lightning task for migrating D2 (https://github.com/facebookresearch/d2go/commit/9d9f438b191634dc38d16f3973e490909b7f67dd)Go's GANsRunner to Lightning based workflow.
The Lightning task could directly work with D2 (https://github.com/facebookresearch/d2go/commit/9d9f438b191634dc38d16f3973e490909b7f67dd)Go e2e workflow.

Reviewed By: tax313

Differential Revision: D28165835

fbshipit-source-id: 4d9d679e188f9d5f9a46f01f7d34a8f30c3e170b

d2go/runner/lightning_task.py

@@ -129,7 +129,7 @@ class DefaultTask(pl.LightningModule):
             self.model_ema = deepcopy(self.model)
             self.dataset_evaluators[ModelTag.EMA] = []
 
-    def _build_model(self):
+    def _build_model(self) -> torch.nn.Module:
         model = build_model(self.cfg)
 
         if self.cfg.MODEL.FROZEN_LAYER_REG_EXP: