[generation] Less verbose warnings by default (#38179)

* tmp commit (imports broken)

* working version; update tests

* remove line break

* shorter msg

* dola checks need num_beams=1; other minor PR comments

* update early trainer failing on bad gen config

* make fixup

* test msg

tests/trainer/test_trainer_seq2seq.py

@@ -202,4 +202,4 @@ class Seq2seqTrainerTester(TestCasePlus):
                 data_collator=data_collator,
                 compute_metrics=lambda x: {"samples": x[0].shape[0]},
             )
-        self.assertIn("The loaded generation config instance is invalid", str(exc.exception))
+        self.assertIn("Fix these issues to train your model", str(exc.exception))