Silence deprecation warning in TensorFlow 2.4 (#810)

opennmt/training.py

@@ -471,7 +471,15 @@ def _finalize_dataset(self, dataset):
         # We prefer not to use experimental_distribute_dataset here because it
         # sometimes fails to split the batches (noticed with tokens batch type).
         dataset_fn = dataset if callable(dataset) else lambda _: dataset
-        return self._strategy.experimental_distribute_datasets_from_function(dataset_fn)
+        # TODO: clean this API usage when TensorFlow requirement is updated to >=2.4.
+        distribute_fn = getattr(
+            self._strategy, "distribute_datasets_from_function", None
+        )
+        if distribute_fn is None:
+            distribute_fn = (
+                self._strategy.experimental_distribute_datasets_from_function
+            )
+        return distribute_fn(dataset_fn)
 
     def _forward(self, source, target, accum_steps=1, report_steps=None):
         per_replica_loss = self._strategy.run(