enable average tokens across devices (huggingface#34373)

* enable average tokens across devices

* reduce earlier in case model needs it

* simplify if statement

* reformat code to make ruff happy

* add doc for argument: average_tokens_across_devices

* cannot find world size when pytorch is unavailable

* format code

---------

Co-authored-by: Zach Mueller <muellerzr@gmail.com>
Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

src/transformers/trainer.py

@@ -3631,7 +3631,12 @@ def training_step(
             with amp.scale_loss(loss, self.optimizer) as scaled_loss:
                 scaled_loss.backward()
         else:
-            loss *= self.args.gradient_accumulation_steps
+            if num_items_in_batch is not None:
+                if self.compute_loss_func or self.model_accepts_loss_kwargs:
+                    loss *= self.args.gradient_accumulation_steps
+                # Average tokens across devices is orthogonal to gradient accumulation
+                if self.args.average_tokens_across_devices:
+                    loss *= self.args.world_size
             self.accelerator.backward(loss, **kwargs)
 
         return loss.detach() / self.args.gradient_accumulation_steps
@@ -3646,6 +3651,9 @@ def compute_loss(self, model, inputs, return_outputs=False, num_items_in_batch=N
             labels = inputs.pop("labels")
         else:
             labels = None
+        if self.args.average_tokens_across_devices and num_items_in_batch is not None:
+            num_items_in_batch_tensor = torch.tensor(num_items_in_batch, device=self.args.device)
+            num_items_in_batch = int(self.accelerator.gather(num_items_in_batch_tensor).sum().cpu())
         if self.model_accepts_loss_kwargs:
             loss_kwargs = {}
             if num_items_in_batch is not None:

src/transformers/training_args.py

@@ -1532,6 +1532,15 @@ class TrainingArguments:
         },
     )
 
+    average_tokens_across_devices: Optional[bool] = field(
+        default=False,
+        metadata={
+            "help": "Whether or not to average tokens across devices. If enabled, will use all_reduce to "
+            "synchronize num_tokens_in_batch for precise loss calculation. Reference: "
+            "https://github.com/huggingface/transformers/issues/34242"
+        },
+    )
+
     def __post_init__(self):
         # Parse in args that could be `dict` sent in from the CLI as a string
         for field in _VALID_DICT_FIELDS:
@@ -1765,6 +1774,19 @@ def __post_init__(self):
         if self.framework == "pt" and is_torch_available():
             self.device
 
+        # Disable average tokens when using single device
+        if self.average_tokens_across_devices:
+            try:
+                if self.world_size == 1:
+                    logger.warning(
+                        "average_tokens_across_devices is set to True but it is invalid when world size is"
+                        "1. Turn it to False automatically."
+                    )
+                    self.average_tokens_across_devices = False
+            except ImportError as e:
+                logger.warning(f"Can not specify world size due to {e}. Turn average_tokens_across_devices to False.")
+                self.average_tokens_across_devices = False
+
         if self.torchdynamo is not None:
             warnings.warn(
                 "`torchdynamo` is deprecated and will be removed in version 5 of 🤗 Transformers. Use"