Enabling multilingual models for translation pipelines. (#10536)

* [WIP] Enabling multilingual models for translation pipelines.

* decoder_input_ids -> forced_bos_token_id

* Improve docstring.

* Rebase

* Fixing 2 bugs

- Type token_ids coming from `_parse_and_tokenize`
- Wrong index from tgt_lang.

* Fixing black version.

* Adding tests for _build_translation_inputs and add them for all
tokenizers.

* Mbart actually puts the lang code at the end.

* Fixing m2m100.

* Adding TF support to `deep_round`.

* Update src/transformers/pipelines/text2text_generation.py

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* Adding one line comment.

* Fixing M2M100 `_build_translation_input_ids`, and fix the call site.

* Fixing tests + deep_round -> nested_simplify

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

tests/test_tokenization_mbart50.py

@@ -17,7 +17,7 @@ import tempfile
 import unittest
 
 from transformers import SPIECE_UNDERLINE, BatchEncoding, MBart50Tokenizer, MBart50TokenizerFast, is_torch_available
-from transformers.testing_utils import require_sentencepiece, require_tokenizers, require_torch
+from transformers.testing_utils import nested_simplify, require_sentencepiece, require_tokenizers, require_torch
 
 from .test_tokenization_common import TokenizerTesterMixin
 
@@ -194,3 +194,18 @@ class MBartOneToManyIntegrationTest(unittest.TestCase):
 
         self.assertEqual(batch.input_ids.shape[1], 3)
         self.assertEqual(batch.decoder_input_ids.shape[1], 10)
+
+    @require_torch
+    def test_tokenizer_translation(self):
+        inputs = self.tokenizer._build_translation_inputs("A test", src_lang="en_XX", tgt_lang="ar_AR")
+
+        self.assertEqual(
+            nested_simplify(inputs),
+            {
+                # en_XX, A, test, EOS
+                "input_ids": [[250004, 62, 3034, 2]],
+                "attention_mask": [[1, 1, 1, 1]],
+                # ar_AR
+                "forced_bos_token_id": 250001,
+            },
+        )