Seq2SeqDataset: avoid passing src_lang everywhere (#7470)

Co-authored-by: Sam Shleifer <sshleifer@gmail.com>

examples/seq2seq/test_datasets.py

@@ -185,3 +185,36 @@ def test_distributed_sortish_sampler_splits_indices_between_procs():
     ids1 = set(DistributedSortishSampler(ds, 256, num_replicas=2, rank=0, add_extra_examples=False))
     ids2 = set(DistributedSortishSampler(ds, 256, num_replicas=2, rank=1, add_extra_examples=False))
     assert ids1.intersection(ids2) == set()
+
+
+@pytest.mark.parametrize(
+    "tok_name",
+    [
+        MBART_TINY,
+        MARIAN_TINY,
+        T5_TINY,
+        BART_TINY,
+        PEGASUS_XSUM,
+    ],
+)
+def test_dataset_kwargs(tok_name):
+    tokenizer = AutoTokenizer.from_pretrained(tok_name)
+    if tok_name == MBART_TINY:
+        train_dataset = Seq2SeqDataset(
+            tokenizer,
+            data_dir=make_test_data_dir(),
+            type_path="train",
+            max_source_length=4,
+            max_target_length=8,
+            src_lang="EN",
+            tgt_lang="FR",
+        )
+        kwargs = train_dataset.dataset_kwargs
+        assert "src_lang" in kwargs and "tgt_lang" in kwargs
+    else:
+        train_dataset = Seq2SeqDataset(
+            tokenizer, data_dir=make_test_data_dir(), type_path="train", max_source_length=4, max_target_length=8
+        )
+        kwargs = train_dataset.dataset_kwargs
+        assert "add_prefix_space" not in kwargs if tok_name != BART_TINY else "add_prefix_space" in kwargs
+        assert len(kwargs) == 1 if tok_name == BART_TINY else len(kwargs) == 0