Add gmadl loss

src/ml/loss.py

@@ -0,0 +1,15 @@
+import torch
+
+from pytorch_forecasting.metrics.base_metrics import MultiHorizonMetric
+
+
+class GMADL(MultiHorizonMetric):
+    """GMADL loss function."""
+
+    def __init__(self, a=1000, b=2, **kwargs):
+        super().__init__(**kwargs)
+        self.a = a
+        self.b = b
+
+    def loss(self, y_pred, target):
+        return -1 * (1 / (1 + torch.exp(-self.a * self.to_prediction(y_pred) * target)) - 0.5) * torch.pow(torch.abs(target), self.b)