Enhancing SFT Training Efficiency Using Packing and FlashAttention2 with Position IDs (#31629)

* add DataCollatorBatchFlattening

* Update data_collator.py

* change name

* new FA2 flow if position_ids is provided

* add comments

* minor fix

* minor fix data collator

* add test cases for models

* add test case for data collator

* remove extra code

* formating for ruff check and check_repo.py

* ruff format

ruff format tests src utils

* custom_init_isort.py

docs/source/en/main_classes/data_collator.md

@@ -66,3 +66,8 @@ Examples of use can be found in the [example scripts](../examples) or [example n
     - numpy_mask_tokens
     - tf_mask_tokens
     - torch_mask_tokens
+
+## DataCollatorWithFlattening
+
+[[autodoc]] data.data_collator.DataCollatorWithFlattening
+