added a if/else block for applying age_batch_norm because there could…

… be batches of single example in the forward pass, which would break the batch_norm

src/cehrbert/models/hf_models/hf_cehrbert.py

@@ -354,6 +354,36 @@ def __init__(self, config: CehrBertConfig):
         # Initialize weights and apply final processing
         self.post_init()
 
+    def _apply_age_norm(
+        self,
+        age_at_index: torch.FloatTensor,
+    ) -> torch.FloatTensor:
+        """
+        Applies batch normalization to the input age tensor.
+
+        If the batch contains more than one example,
+        standard batch normalization is applied. If the batch size is 1, batch normalization is applied
+        without updating the running statistics, ensuring that the normalization uses the stored running
+        mean and variance without modification.
+
+        Args:
+            age_at_index (torch.FloatTensor): A tensor containing the age values to normalize.
+            The tensor has shape `(batch_size, num_features)` where `batch_size` is the number of samples in the batch.
+
+        Returns:
+            torch.FloatTensor: A tensor with the normalized age values.
+        """
+        if age_at_index.shape[0] > 1:
+            normalized_age = self.age_batch_norm(age_at_index)
+        else:
+            self.age_batch_norm.eval()
+            # Apply batch norm without updating running stats
+            with torch.no_grad():  # Prevent tracking gradients, since we don't want to update anything
+                normalized_age = self.age_batch_norm(age_at_index)
+            # Optionally, set the layer back to training mode if needed later
+            self.age_batch_norm.train()
+        return normalized_age
+
     def forward(
         self,
         input_ids: torch.LongTensor,
@@ -369,7 +399,8 @@ def forward(
         output_hidden_states: Optional[bool] = None,
         classifier_label: Optional[torch.FloatTensor] = None,
     ) -> CehrBertSequenceClassifierOutput:
-        normalized_age = self.age_batch_norm(age_at_index)
+
+        normalized_age = self._apply_age_norm(age_at_index)
 
         cehrbert_output = self.bert(
             input_ids,