Add Flash Attention 2 to M2M100 model (#30256)

* Added flash attention 2.

* Fixes.

* Fix inheritance.

* Fixed init.

* Remove stuff.

* Added documentation.

* Add FA2 to M2M100 documentation.

* Add test.

* Fixed documentation.

* Update src/transformers/models/m2m_100/modeling_m2m_100.py

Co-authored-by: Younes Belkada <49240599+younesbelkada@users.noreply.github.com>

* Update docs/source/en/model_doc/nllb.md

Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

* Fixed variable name.

---------

Co-authored-by: Younes Belkada <49240599+younesbelkada@users.noreply.github.com>
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

tests/models/m2m_100/test_modeling_m2m_100.py

@@ -19,12 +19,16 @@ import copy
 import tempfile
 import unittest
 
+import pytest
+
 from transformers import M2M100Config, is_torch_available
 from transformers.testing_utils import (
+    require_flash_attn,
     require_sentencepiece,
     require_tokenizers,
     require_torch,
     require_torch_fp16,
+    require_torch_gpu,
     slow,
     torch_device,
 )
@@ -412,3 +416,48 @@ class M2M100ModelIntegrationTests(unittest.TestCase):
             hypotheses_batch.tolist(), clean_up_tokenization_spaces=True, skip_special_tokens=True
         )
         assert generated == expected_en
+
+    @require_flash_attn
+    @require_torch_gpu
+    @pytest.mark.flash_attn_test
+    @slow
+    def test_flash_attn_2_seq_to_seq_generation(self):
+        """
+        Overwritting the common test as the test is flaky on tiny models
+        """
+        model = M2M100ForConditionalGeneration.from_pretrained(
+            "facebook/m2m100_418M", attn_implementation="flash_attention_2"
+        ).to(torch_device)
+
+        tokenizer = M2M100Tokenizer.from_pretrained("facebook/m2m100_418M", src_lang="fr", tgt_lang="en")
+
+        src_fr = [
+            "L'affaire NSA souligne l'absence totale de débat sur le renseignement",
+            "Selon moi, il y a deux niveaux de réponse de la part du gouvernement français.",
+            "Lorsque François Hollande téléphone à Barack Obama ou quand le ministre des affaires étrangères Laurent"
+            " Fabius convoque l'ambassadeur des Etats-Unis, ils réagissent à une vraie découverte, qui est celle de"
+            " l'ampleur de la surveillance américaine sur l'ensemble des communications en France.",
+        ]
+
+        # The below article tests that we don't add any hypotheses outside of the top n_beams
+        dct = tokenizer(src_fr, padding=True, return_tensors="pt")
+
+        hypotheses_batch = model.generate(
+            input_ids=dct["input_ids"].to(torch_device),
+            attention_mask=dct["attention_mask"].to(torch_device),
+            num_beams=5,
+            forced_bos_token_id=tokenizer.get_lang_id("en"),
+        )
+
+        expected_en = [
+            "The NSA case highlights the total absence of intelligence debate",
+            "I think there are two levels of response from the French government.",
+            "When François Hollande calls Barack Obama or when Foreign Minister Laurent Fabius calls the U.S."
+            " Ambassador, they respond to a real discovery, which is that of the scale of U.S. surveillance on all"
+            " communications in France.",
+        ]
+
+        generated = tokenizer.batch_decode(
+            hypotheses_batch.tolist(), clean_up_tokenization_spaces=True, skip_special_tokens=True
+        )
+        assert generated == expected_en