[feat] truncation support with excess_length_strategy (#3068) [skip ci]

* feat:truncation support with excess_len

* pre-commit

* excess_length_strategy

* requested changes

* lint

* added handle_long_seq_in_dataset in sft

* comments improved

src/axolotl/utils/data/sft.py

@@ -28,7 +28,7 @@ from axolotl.utils.data.shared import (
 )
 from axolotl.utils.data.utils import (
     deduplicate_and_log_datasets,
-    drop_long_seq_in_dataset,
+    handle_long_seq_in_dataset,
     retry_on_request_exceptions,
 )
 from axolotl.utils.data.wrappers import get_dataset_wrapper
@@ -339,9 +339,9 @@ def _load_raw_datasets(
 
     if not cfg.skip_prepare_dataset:
         if split == "test" and cfg.eval_sequence_len:
-            dataset = drop_long_seq_in_dataset(dataset, cfg.eval_sequence_len, cfg)
+            dataset = handle_long_seq_in_dataset(dataset, cfg.eval_sequence_len, cfg)
         else:
-            dataset = drop_long_seq_in_dataset(dataset, cfg.sequence_len, cfg)
+            dataset = handle_long_seq_in_dataset(dataset, cfg.sequence_len, cfg)
         if cfg.sample_packing:
             dataset, _ = process_datasets_for_packing(cfg, dataset, None)
 

src/axolotl/utils/data/utils.py

@@ -148,7 +148,36 @@ def deduplicate_and_log_datasets(
     return dataset, other_dataset
 
 
-def drop_long_seq_in_dataset(
+def truncate_long_seq(sample, sequence_len=2048, min_sequence_len=2):
+    """
+    Truncate samples whose sequence length is too long (> sequence_len)
+    or drop those too short (< min_sequence_len).
+    """
+    min_sequence_len = min_sequence_len or 2
+
+    input_ids = sample["input_ids"]
+    results = []
+
+    # Batched (input_ids is a list of lists)
+    for i, seq in enumerate(input_ids):
+        length = len(seq)
+        if length < min_sequence_len:
+            results.append(False)
+        elif length > sequence_len:
+            sample["input_ids"][i] = seq[:sequence_len]
+            if "attention_mask" in sample:
+                sample["attention_mask"][i] = sample["attention_mask"][i][:sequence_len]
+            if "labels" in sample:
+                sample["labels"][i] = sample["labels"][i][:sequence_len]
+            if "position_ids" in sample:
+                sample["position_ids"][i] = sample["position_ids"][i][:sequence_len]
+            results.append(True)
+        else:
+            results.append(True)
+    return results
+
+
+def handle_long_seq_in_dataset(
     dataset: Dataset, sequence_len: int, cfg: DictDefault
 ) -> Dataset:
     """Remove sequences longer than configured maximum from dataset.
@@ -192,8 +221,21 @@ def drop_long_seq_in_dataset(
     if filter_map_kwargs:
         drop_long_kwargs["desc"] = f"Dropping Long Sequences (>{sequence_len})"
 
+    excess_length_strategy = (cfg.excess_length_strategy or "drop").lower()
+    if excess_length_strategy == "truncate":
+        process_fn = functools.partial(
+            truncate_long_seq,
+            sequence_len=sequence_len,
+            min_sequence_len=cfg.min_sample_len,
+        )
+        drop_long_kwargs["desc"] = (
+            f"Truncating/Filtering Sequences (target_len={sequence_len})"
+        )
+    else:
+        process_fn = drop_long
+
     dataset = dataset.filter(
-        drop_long,
+        process_fn,
         batched=True,
         **filter_map_kwargs,
         **drop_long_kwargs,
@@ -201,6 +243,11 @@ def drop_long_seq_in_dataset(
     if prior_len:
         dropped = prior_len - len(dataset)
         if dropped:
-            LOG.warning(f"Dropped {dropped} long samples from dataset")
+            action = (
+                "truncated/filtered"
+                if excess_length_strategy == "truncate"
+                else "dropped"
+            )
+            LOG.warning(f"{action.title()} {dropped} samples from dataset")
 
     return dataset

src/axolotl/utils/schemas/config.py

@@ -414,6 +414,12 @@ class AxolotlInputConfig(
             "description": "The maximum length of an input to train with, this should typically be less than 2048 as most models have a token/context limit of 2048"
         },
     )
+    excess_length_strategy: Literal["drop", "truncate"] | None = Field(
+        default=None,
+        json_schema_extra={
+            "description": "What to do when a tokenized row exceeds sequence_len. 'drop' removes the row; 'truncate' slices tensors to sequence_len. Defaults to 'drop' for backward compatibility."
+        },
+    )
     eval_sequence_len: int | None = Field(
         default=None,
         json_schema_extra={

tests/test_packed_batch_sampler.py

@@ -8,7 +8,7 @@ from transformers import AutoTokenizer
 from axolotl.datasets import TokenizedPromptDataset
 from axolotl.prompt_strategies.completion import load
 from axolotl.utils.collators import V2BatchSamplerDataCollatorForSeq2Seq
-from axolotl.utils.data.utils import drop_long_seq_in_dataset
+from axolotl.utils.data.utils import handle_long_seq_in_dataset
 from axolotl.utils.dict import DictDefault
 from axolotl.utils.samplers import MultipackBatchSampler, get_dataset_lengths
 
@@ -70,7 +70,7 @@ class TestBatchedSamplerPacking:
         )
         train_dataset = concatenate_datasets([dataset_wrapper])
 
-        train_dataset = drop_long_seq_in_dataset(train_dataset, cfg.sequence_len, cfg)
+        train_dataset = handle_long_seq_in_dataset(train_dataset, cfg.sequence_len, cfg)
 
         lengths = get_dataset_lengths(train_dataset)
         batch_sampler = MultipackBatchSampler(