Implement Explained Variance Metric + metric fix (#4013)

* metric fix, explained variance

* one more test

* pep8

* remove comment

* fix add_state condition

Co-authored-by: ananyahjha93 <ananya@pytorchlightning.ai>

docs/source/metrics.rst

@@ -181,6 +181,13 @@ MeanSquaredLogError
     :noindex:
 
 
+ExplainedVariance
+^^^^^^^^^^^^^^^^^
+
+.. autoclass:: pytorch_lightning.metrics.regression.ExplainedVariance
+    :noindex:
+
+
 Functional Metrics
 ==================
 

pytorch_lightning/metrics/__init__.py

@@ -1,4 +1,9 @@
 from pytorch_lightning.metrics.metric import Metric
 
 from pytorch_lightning.metrics.classification.accuracy import Accuracy
-from pytorch_lightning.metrics.regression import MeanSquaredError, MeanAbsoluteError, MeanSquaredLogError
+from pytorch_lightning.metrics.regression import (
+    MeanSquaredError,
+    MeanAbsoluteError,
+    MeanSquaredLogError,
+    ExplainedVariance,
+)

pytorch_lightning/metrics/metric.py

@@ -96,7 +96,11 @@ class Metric(nn.Module, ABC):
             the format discussed in the above note.
 
         """
-        if not isinstance(default, torch.Tensor) or (isinstance(default, list) and len(default) != 0):
+        if (
+            not isinstance(default, torch.Tensor)
+            and not isinstance(default, list)                     # noqa: W503
+            or (isinstance(default, list) and len(default) != 0)  # noqa: W503
+        ):
             raise ValueError(
                 "state variable must be a tensor or any empty list (where you can append tensors)"
             )
@@ -163,7 +167,7 @@ class Metric(nn.Module, ABC):
             elif isinstance(output_dict[attr][0], list):
                 output_dict[attr] = _flatten(output_dict[attr])
 
-            assert isinstance(reduction_fn, (Callable, None))
+            assert isinstance(reduction_fn, (Callable)) or reduction_fn is None
             reduced = reduction_fn(output_dict[attr]) if reduction_fn is not None else output_dict[attr]
             setattr(self, attr, reduced)
 

pytorch_lightning/metrics/regression/__init__.py

@@ -1,3 +1,4 @@
 from pytorch_lightning.metrics.regression.mean_squared_error import MeanSquaredError
 from pytorch_lightning.metrics.regression.mean_absolute_error import MeanAbsoluteError
 from pytorch_lightning.metrics.regression.mean_squared_log_error import MeanSquaredLogError
+from pytorch_lightning.metrics.regression.explained_variance import ExplainedVariance

pytorch_lightning/metrics/regression/explained_variance.py

@@ -0,0 +1,63 @@
+import torch
+from typing import Any, Callable, Optional, Union
+
+from pytorch_lightning.metrics.metric import Metric
+
+
+class ExplainedVariance(Metric):
+    """
+    Computes explained variance.
+
+    Example:
+
+        >>> from pytorch_lightning.metrics import ExplainedVariance
+        >>> target = torch.tensor([3, -0.5, 2, 7])
+        >>> preds = torch.tensor([2.5, 0.0, 2, 8])
+        >>> explained_variance = ExplainedVariance()
+        >>> explained_variance(preds, target)
+        tensor(0.9572)
+
+
+    """
+
+    def __init__(
+        self,
+        compute_on_step: bool = True,
+        ddp_sync_on_step: bool = False,
+        process_group: Optional[Any] = None,
+    ):
+        super().__init__(
+            compute_on_step=compute_on_step,
+            ddp_sync_on_step=ddp_sync_on_step,
+            process_group=process_group,
+        )
+
+        self.add_state("y", default=[], dist_reduce_fx=None)
+        self.add_state("y_pred", default=[], dist_reduce_fx=None)
+
+    def update(self, preds: torch.Tensor, target: torch.Tensor):
+        """
+        Update state with predictions and targets.
+
+        Args:
+            preds: Predictions from model
+            target: Ground truth values
+        """
+        self.y.append(target)
+        self.y_pred.append(preds)
+
+    def compute(self):
+        """
+        Computes explained variance over state.
+        """
+        y_true = torch.cat(self.y, dim=0)
+        y_pred = torch.cat(self.y_pred, dim=0)
+
+        y_diff_avg = torch.mean(y_true - y_pred, dim=0)
+        numerator = torch.mean((y_true - y_pred - y_diff_avg) ** 2, dim=0)
+
+        y_true_avg = torch.mean(y_true, dim=0)
+        denominator = torch.mean((y_true - y_true_avg) ** 2, dim=0)
+
+        # TODO: multioutput
+        return 1.0 - torch.mean(numerator / denominator)

tests/metrics/regression/test_explained_variance.py

@@ -0,0 +1,51 @@
+import torch
+import pytest
+from collections import namedtuple
+from functools import partial
+
+from pytorch_lightning.metrics.regression import ExplainedVariance
+from sklearn.metrics import explained_variance_score
+
+from tests.metrics.utils import compute_batch, setup_ddp
+from tests.metrics.utils import NUM_BATCHES, NUM_PROCESSES, BATCH_SIZE
+
+torch.manual_seed(42)
+
+num_targets = 5
+
+Input = namedtuple('Input', ["preds", "target"])
+
+_single_target_inputs = Input(
+    preds=torch.rand(NUM_BATCHES, BATCH_SIZE),
+    target=torch.rand(NUM_BATCHES, BATCH_SIZE),
+)
+
+_multi_target_inputs = Input(
+    preds=torch.rand(NUM_BATCHES, BATCH_SIZE, num_targets),
+    target=torch.rand(NUM_BATCHES, BATCH_SIZE, num_targets),
+)
+
+
+def _single_target_sk_metric(preds, target, sk_fn=explained_variance_score):
+    sk_preds = preds.view(-1).numpy()
+    sk_target = target.view(-1).numpy()
+    return sk_fn(sk_target, sk_preds)
+
+
+def _multi_target_sk_metric(preds, target, sk_fn=explained_variance_score):
+    sk_preds = preds.view(-1, num_targets).numpy()
+    sk_target = target.view(-1, num_targets).numpy()
+    return sk_fn(sk_target, sk_preds)
+
+
+@pytest.mark.parametrize("ddp", [True, False])
+@pytest.mark.parametrize("ddp_sync_on_step", [True, False])
+@pytest.mark.parametrize(
+    "preds, target, sk_metric",
+    [
+        (_single_target_inputs.preds, _single_target_inputs.target, _single_target_sk_metric),
+        (_multi_target_inputs.preds, _multi_target_inputs.target, _multi_target_sk_metric),
+    ],
+)
+def test_explained_variance(ddp, ddp_sync_on_step, preds, target, sk_metric):
+    compute_batch(preds, target, ExplainedVariance, sk_metric, ddp_sync_on_step, ddp)