📦 Packing documentation (#2503)

docs/source/reducing_memory_usage.md

@@ -51,4 +51,37 @@ training_args = SFTConfig(..., max_length=...)
 ```
 
 </hfoption>
-</hfoptions>
+</hfoptions>
+
+## Packing
+
+<Tip>
+
+This technique applies only to SFT.
+
+</Tip>
+
+
+[Truncation](#truncation) has several drawbacks:
+1. **Loss of information**: Key data at the end of a sequence may be discarded.
+2. **Choosing truncation length**: Too short loses data; too long undermines efficiency.
+
+Packing, introduced in [Raffel et al., 2020](https://huggingface.co/papers/1910.10683), addresses these issues by grouping sequences instead of truncating. It concatenates and splits dataset sequences into the desired lengths.
+
+<div class="flex justify-center">
+    <img src="https://huggingface.co/datasets/trl-lib/documentation-images/resolve/main/packing.png" alt="Packing" width="600"/>
+</div>
+
+Packing eliminates padding, preserves all sequence information, and allows for flexible sequence lengths, making it a more efficient alternative to truncation. To enable packing, use `packing=True` in the [`SFTConfig`]:
+
+```python
+from trl import SFTConfig
+
+training_args = SFTConfig(..., packing=True, max_seq_length=512)
+```
+
+<Tip warning={true}>
+
+Packing may cause batch contamination, where adjacent sequences influence one another. This can be problematic for some applications. For more details, see [#1230](https://github.com/huggingface/trl/issues/1230).
+
+</Tip>