Fixing the "translation", "translation_XX_to_YY" pipelines. (#7975)

* Actually make the "translation", "translation_XX_to_YY" task behave correctly.

Background:
- Currently "translation_cn_to_ar" does not work. (only 3 pairs are
supported)
- Some models, contain in their config the correct values for the (src,
tgt) pair they can translate. It's usually just one pair, and we can
infer it automatically from the `model.config.task_specific_params`. If
it's not defined we can still probably load the TranslationPipeline
nevertheless.

Proposed fix:
- A simplified version of what could become more general which is
a `parametrized` task. "translation" + (src, tgt) in this instance
it what we need in the general case. The way we go about it for now
is simply parsing "translation_XX_to_YY". If cases of parametrized task arise
we should preferably go in something closer to what `datasets` propose
which is having a secondary argument `task_options`? that will be close
to what that task requires.
- Should be backward compatible in all cases for instance
`pipeline(task="translation_en_to_de") should work out of the box.
- Should provide a warning when a specific translation pair has been
selected on behalf of the user using
`model.config.task_specific_params`.

* Update src/transformers/pipelines.py

Co-authored-by: Julien Chaumond <chaumond@gmail.com>

Co-authored-by: Julien Chaumond <chaumond@gmail.com>

tests/test_pipelines.py

@@ -1,6 +1,8 @@
 import unittest
 from typing import Iterable, List, Optional
 
+import pytest
+
 from transformers import pipeline
 from transformers.pipelines import SUPPORTED_TASKS, Conversation, DefaultArgumentHandler, Pipeline
 from transformers.testing_utils import require_tf, require_tokenizers, require_torch, slow, torch_device
@@ -392,6 +394,33 @@ class MonoColumnInputTestCase(unittest.TestCase):
                 invalid_inputs,
             )
 
+    @require_torch
+    @slow
+    def test_default_translations(self):
+        # We don't provide a default for this pair
+        with self.assertRaises(ValueError):
+            pipeline(task="translation_cn_to_ar")
+
+        # but we do for this one
+        pipeline(task="translation_en_to_de")
+
+    @require_torch
+    def test_translation_on_odd_language(self):
+        model = TRANSLATION_FINETUNED_MODELS[0][0]
+        pipeline(task="translation_cn_to_ar", model=model)
+
+    @require_torch
+    def test_translation_default_language_selection(self):
+        model = TRANSLATION_FINETUNED_MODELS[0][0]
+        with pytest.warns(UserWarning, match=r".*translation_en_to_de.*"):
+            nlp = pipeline(task="translation", model=model)
+        self.assertEqual(nlp.task, "translation_en_to_de")
+
+    @require_torch
+    def test_translation_with_no_language_no_model_fails(self):
+        with self.assertRaises(ValueError):
+            pipeline(task="translation")
+
     @require_tf
     @slow
     def test_tf_translation(self):