add action_out abstract method and docstring

ml-agents/mlagents/trainers/torch/distributions.py

         """
         pass
 
+    @abc.abstractmethod
+    def action_out(self) -> torch.Tensor:
+        """
+        Returns the tensor to be exported to ONNX for the distribution
+        """
+        pass
+
 
 class DiscreteDistInstance(DistInstance):
     @abc.abstractmethod