Make num_classes optional, in case of micro averaging

CHANGELOG.md

@@ -17,6 +17,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- Make `num_classes` optional for classification in case of micro averaging ([#2841](https://github.com/PyTorchLightning/metrics/pull/2841))
+
+
 - Enabled specifying weights path for FID ([#2867](https://github.com/PyTorchLightning/metrics/pull/2867))
 
 

src/torchmetrics/classification/stat_scores.py

@@ -307,7 +307,7 @@ class MulticlassStatScores(_AbstractStatScores):
 
     def __init__(
         self,
-        num_classes: int,
+        num_classes: Optional[int] = None,
         top_k: int = 1,
         average: Optional[Literal["micro", "macro", "weighted", "none"]] = "macro",
         multidim_average: Literal["global", "samplewise"] = "global",
@@ -330,7 +330,7 @@ def __init__(
         self.zero_division = zero_division
 
         self._create_state(
-            size=1 if (average == "micro" and top_k == 1) else num_classes, multidim_average=multidim_average
+            size=1 if (average == "micro" and top_k == 1) else (num_classes or 1), multidim_average=multidim_average
         )
 
     def update(self, preds: Tensor, target: Tensor) -> None:
@@ -340,8 +340,9 @@ def update(self, preds: Tensor, target: Tensor) -> None:
                 preds, target, self.num_classes, self.multidim_average, self.ignore_index
             )
         preds, target = _multiclass_stat_scores_format(preds, target, self.top_k)
+        num_classes = self.num_classes if self.num_classes is not None else 1
         tp, fp, tn, fn = _multiclass_stat_scores_update(
-            preds, target, self.num_classes, self.top_k, self.average, self.multidim_average, self.ignore_index
+            preds, target, num_classes, self.top_k, self.average, self.multidim_average, self.ignore_index
         )
         self._update_state(tp, fp, tn, fn)
 

src/torchmetrics/functional/classification/accuracy.py

@@ -166,7 +166,7 @@ def binary_accuracy(
 def multiclass_accuracy(
     preds: Tensor,
     target: Tensor,
-    num_classes: int,
+    num_classes: Optional[int] = None,
     average: Optional[Literal["micro", "macro", "weighted", "none"]] = "macro",
     top_k: int = 1,
     multidim_average: Literal["global", "samplewise"] = "global",
@@ -266,7 +266,7 @@ def multiclass_accuracy(
         _multiclass_stat_scores_tensor_validation(preds, target, num_classes, multidim_average, ignore_index)
     preds, target = _multiclass_stat_scores_format(preds, target, top_k)
     tp, fp, tn, fn = _multiclass_stat_scores_update(
-        preds, target, num_classes, top_k, average, multidim_average, ignore_index
+        preds, target, num_classes or 1, top_k, average, multidim_average, ignore_index
     )
     return _accuracy_reduce(tp, fp, tn, fn, average=average, multidim_average=multidim_average, top_k=top_k)
 

src/torchmetrics/functional/classification/stat_scores.py

@@ -218,7 +218,7 @@ def binary_stat_scores(
 
 
 def _multiclass_stat_scores_arg_validation(
-    num_classes: int,
+    num_classes: Optional[int],
     top_k: int = 1,
     average: Optional[Literal["micro", "macro", "weighted", "none"]] = "macro",
     multidim_average: Literal["global", "samplewise"] = "global",
@@ -235,11 +235,11 @@ def _multiclass_stat_scores_arg_validation(
     - ``zero_division`` has to be 0 or 1
 
     """
-    if not isinstance(num_classes, int) or num_classes < 2:
+    if num_classes is not None and (not isinstance(num_classes, int) or num_classes < 2):
         raise ValueError(f"Expected argument `num_classes` to be an integer larger than 1, but got {num_classes}")
     if not isinstance(top_k, int) and top_k < 1:
         raise ValueError(f"Expected argument `top_k` to be an integer larger than or equal to 1, but got {top_k}")
-    if top_k > num_classes:
+    if top_k > (num_classes if num_classes is not None else 1):
         raise ValueError(
             f"Expected argument `top_k` to be smaller or equal to `num_classes` but got {top_k} and {num_classes}"
         )
@@ -260,7 +260,7 @@ def _multiclass_stat_scores_arg_validation(
 def _multiclass_stat_scores_tensor_validation(
     preds: Tensor,
     target: Tensor,
-    num_classes: int,
+    num_classes: Optional[int],
     multidim_average: Literal["global", "samplewise"] = "global",
     ignore_index: Optional[int] = None,
 ) -> None:
@@ -278,7 +278,7 @@ def _multiclass_stat_scores_tensor_validation(
     if preds.ndim == target.ndim + 1:
         if not preds.is_floating_point():
             raise ValueError("If `preds` have one dimension more than `target`, `preds` should be a float tensor.")
-        if preds.shape[1] != num_classes:
+        if num_classes is not None and preds.shape[1] != num_classes:
             raise ValueError(
                 "If `preds` have one dimension more than `target`, `preds.shape[1]` should be"
                 " equal to number of classes."
@@ -310,15 +310,15 @@ def _multiclass_stat_scores_tensor_validation(
             "Either `preds` and `target` both should have the (same) shape (N, ...), or `target` should be (N, ...)"
             " and `preds` should be (N, C, ...)."
         )
-
-    check_value = num_classes if ignore_index is None else num_classes + 1
-    for t, name in ((target, "target"),) + ((preds, "preds"),) if not preds.is_floating_point() else ():  # noqa: RUF005
-        num_unique_values = len(torch.unique(t, dim=None))
-        if num_unique_values > check_value:
-            raise RuntimeError(
-                f"Detected more unique values in `{name}` than expected. Expected only {check_value} but found"
-                f" {num_unique_values} in `{name}`. Found values: {torch.unique(t, dim=None)}."
-            )
+    if num_classes is not None:
+        check_value = num_classes if ignore_index is None else num_classes + 1
+        for t, name in ((target, "target"),) + ((preds, "preds"),) if not preds.is_floating_point() else ():  # noqa: RUF005
+            num_unique_values = len(torch.unique(t, dim=None))
+            if num_unique_values > check_value:
+                raise RuntimeError(
+                    f"Detected more unique values in `{name}` than expected. Expected only {check_value} but found"
+                    f" {num_unique_values} in `{name}`. Found values: {torch.unique(t, dim=None)}."
+                )
 
 
 def _multiclass_stat_scores_format(

tests/unittests/classification/test_accuracy.py

@@ -626,6 +626,12 @@ def test_corner_cases():
     res = metric(preds, target)
     assert res == 1.0
 
+    metric_micro1 = MulticlassAccuracy(num_classes=None, average="micro", ignore_index=0)
+    metric_micro2 = MulticlassAccuracy(num_classes=3, average="micro", ignore_index=0)
+    res1 = metric_micro1(preds, target)
+    res2 = metric_micro2(preds, target)
+    assert res1 == res2
+
 
 @pytest.mark.parametrize(
     ("metric", "kwargs"),