fix: issue #477 (#478)

## Description


Fix issue  #477. Method shape missing from MissingDates

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Author: b8raoult

src/anemoi/datasets/data/missing.py

@@ -440,3 +440,8 @@ def forwards_subclass_metadata_specific(self) -> dict[str, Any]:
             Metadata specific to the subclass.
         """
         return {"start": self.start, "end": self.end}
+
+    @property
+    def shape(self) -> tuple[int, ...]:
+        """Return the shape of the dataset."""
+        return (len(self),) + self.forward.shape[1:]