add a unit test

darts/tests/models/forecasting/test_probabilistic_models.py

@@ -242,3 +242,24 @@ def _get_avgs(series):
                     "The difference between the mean forecast and the mean series is larger "
                     "than expected on component 1 for distribution {}".format(lkl),
                 )
+
+        def test_stochastic_inputs(self):
+            model = RNNModel(input_chunk_length=5)
+            model.fit(self.constant_ts, epochs=2)
+
+            # build a stochastic series
+            target_vals = self.constant_ts.values()
+            stochastic_vals = np.random.normal(
+                loc=target_vals, scale=1.0, size=(len(self.constant_ts), 100)
+            )
+            stochastic_vals = np.expand_dims(stochastic_vals, axis=1)
+            stochastic_series = TimeSeries.from_times_and_values(
+                self.constant_ts.time_index, stochastic_vals
+            )
+
+            # A deterministic model forecasting a stochastic series
+            # should return stochastic samples
+            preds = [model.predict(series=stochastic_series, n=10) for _ in range(2)]
+
+            # random samples should differ
+            self.assertFalse(np.alltrue(preds[0].values() == preds[1].values()))