Fix amp autocast

CHANGELOG.md

@@ -21,6 +21,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Fixed
 
+- Fixed incorrect yield logic for the amp autocast context manager ([#6080](https://github.com/PyTorchLightning/pytorch-lightning/pull/6080))
+
 
 ## [1.2.0] - 2021-02-18
 

pytorch_lightning/plugins/precision/native_amp.py

@@ -91,4 +91,5 @@ def post_optimizer_step(self, optimizer: Optimizer, optimizer_idx: int) -> None:
     @contextmanager
     def train_step_context(self) -> Generator[autocast, None, None]:
         """Enable autocast context"""
-        yield torch.cuda.amp.autocast()
+        with torch.cuda.amp.autocast():
+            yield

tests/models/test_amp.py

@@ -27,6 +27,16 @@
 from tests.helpers import BoringModel
 
 
+class AMPTestModel(BoringModel):
+
+    def training_step(self, batch, batch_idx):
+        assert torch.is_autocast_enabled()
+        output = self(batch)
+        assert output.dtype == torch.float16
+        loss = self.loss(batch, output)
+        return {"loss": loss}
+
+
 @pytest.mark.skip(reason='dp + amp not supported currently')  # TODO
 @pytest.mark.skipif(not torch.cuda.is_available(), reason="test requires GPU machine")
 def test_amp_single_gpu_dp(tmpdir):
@@ -41,7 +51,7 @@ def test_amp_single_gpu_dp(tmpdir):
         precision=16,
     )
 
-    model = BoringModel()
+    model = AMPTestModel()
     # tutils.run_model_test(trainer_options, model)
     trainer.fit(model)
 
@@ -60,10 +70,9 @@ def test_amp_single_gpu_ddp_spawn(tmpdir):
         precision=16,
     )
 
-    model = BoringModel()
+    model = AMPTestModel()
     # tutils.run_model_test(trainer_options, model)
     trainer.fit(model)
-
     assert trainer.state == TrainerState.FINISHED, f"Training failed with {trainer.state}"
 
 
@@ -81,7 +90,7 @@ def test_amp_multi_gpu_dp(tmpdir):
         precision=16,
     )
 
-    model = BoringModel()
+    model = AMPTestModel()
     # tutils.run_model_test(trainer_options, model)
     trainer.fit(model)
 
@@ -100,10 +109,9 @@ def test_amp_multi_gpu_ddp_spawn(tmpdir):
         precision=16,
     )
 
-    model = BoringModel()
+    model = AMPTestModel()
     # tutils.run_model_test(trainer_options, model)
     trainer.fit(model)
-
     assert trainer.state == TrainerState.FINISHED, f"Training failed with {trainer.state}"
 
 
@@ -122,7 +130,7 @@ def test_amp_gpu_ddp_slurm_managed(tmpdir):
     # simulate setting slurm flags
     tutils.set_random_master_port()
 
-    model = BoringModel()
+    model = AMPTestModel()
 
     # exp file to get meta
     logger = tutils.get_default_logger(tmpdir)