Avoid torchscript export for Metric forward (#4428)

* Update metric.py

* add test

* Update CHANGELOG.md

* Update test_metric_lightning.py

* Update test_metric_lightning.py

Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>

CHANGELOG.md

@@ -19,7 +19,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Added timeout for `tpu_device_exists` to ensure process does not hang indefinitely ([#4340](https://github.com/PyTorchLightning/pytorch-lightning/pull/4340))
 
-- Added global step indexing to the checkpoint name for a better sub-epoch checkpointing experience ([#3807](https://github.com/PyTorchLightning/pytorch-lightning/pull/3807)) 
+- Added global step indexing to the checkpoint name for a better sub-epoch checkpointing experience ([#3807](https://github.com/PyTorchLightning/pytorch-lightning/pull/3807))
 
 ### Changed
 
@@ -47,6 +47,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Fixed AMP unscale for `on_after_backward` ([#4439](https://github.com/PyTorchLightning/pytorch-lightning/pull/4439))
 
+- Fixed TorchScript export when module includes Metrics ([#4428](https://github.com/PyTorchLightning/pytorch-lightning/pull/4428))
 
 ## [1.0.4] - 2020-10-27
 

pytorch_lightning/metrics/metric.py

@@ -145,6 +145,7 @@ class Metric(nn.Module, ABC):
         self._defaults[name] = deepcopy(default)
         self._reductions[name] = dist_reduce_fx
 
+    @torch.jit.unused
     def forward(self, *args, **kwargs):
         """
         Automatically calls ``update()``. Returns the metric value over inputs if ``compute_on_step`` is True.

tests/metrics/test_metric_lightning.py

@@ -1,5 +1,6 @@
-import torch
+import os
 
+import torch
 from pytorch_lightning import Trainer
 from pytorch_lightning.metrics import Metric
 from tests.base.boring_model import BoringModel
@@ -78,3 +79,41 @@ def test_metric_lightning_log(tmpdir):
 
     logged = trainer.logged_metrics
     assert torch.allclose(torch.tensor(logged["sum"]), model.sum)
+
+
+def test_scriptable(tmpdir):
+    class TestModel(BoringModel):
+        def __init__(self):
+            super().__init__()
+            # the metric is not used in the module's `forward`
+            # so the module should be exportable to TorchScript
+            self.metric = SumMetric()
+            self.sum = 0.0
+
+        def training_step(self, batch, batch_idx):
+            x = batch
+            self.metric(x.sum())
+            self.sum += x.sum()
+            self.log("sum", self.metric, on_epoch=True, on_step=False)
+            return self.step(x)
+
+    model = TestModel()
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        limit_train_batches=2,
+        limit_val_batches=2,
+        max_epochs=1,
+        log_every_n_steps=1,
+        weights_summary=None,
+        logger=False,
+        checkpoint_callback=False,
+    )
+    trainer.fit(model)
+    rand_input = torch.randn(10, 32)
+
+    script_model = model.to_torchscript()
+
+    # test that we can still do inference
+    output = model(rand_input)
+    script_output = script_model(rand_input)
+    assert torch.allclose(output, script_output)