Loss

BERTMLMLossWithReduction

Bases: _Nemo2CompatibleLossReduceMixin, MegatronLossReduction

Source code in bionemo/llm/model/loss.py

class BERTMLMLossWithReduction(_Nemo2CompatibleLossReduceMixin, MegatronLossReduction):  # noqa: D101
    def __init__(
        self,
        validation_step: bool = False,
        val_drop_last: bool = True,
    ) -> None:
        """Initializes the Model class.

        Args:
            validation_step (bool, optional): Whether this object is being applied to the validation step. Defaults to False.
            val_drop_last (bool, optional): Whether the last batch is configured to be dropped during validation. Defaults to True.
        """
        # TODO(@jomitchell): Track down how we handle test. This is a common pattern in NeMo2, but these parameters seem likely
        #  to change in the future.
        super().__init__()
        self.validation_step = validation_step
        self.val_drop_last = val_drop_last

    def forward(
        self, batch: Dict[str, Tensor], forward_out: Dict[str, Tensor]
    ) -> Tuple[Tensor, PerTokenLossDict | SameSizeLossDict | DataParallelGroupLossAndIO]:
        """Computes loss of `labels` in the batch vs `token_logits` in the forward output currently. In the future this will be extended
            to handle other loss types like sequence loss if it is present in the forward_out and batch.

        Args:
            batch (Dict[str, Tensor]): The batch of data. Each tensor should be of shape [batch_size, *, *],
                and match the corresponding dimension for that particular key in the batch output.
                For example, the "labels" and "token_logits" key should have a tensor of shape [batch_size, sequence_length].
            forward_out (Dict[str, Tensor]): The forward output from the model. Each tensor should be of shape [batch_size, *, *]

        Taken from:
        https://github.com/NVIDIA/NeMo/blob/main/nemo/collections/nlp/models/language_modeling/megatron_gpt_model.py#L951-L976 .
        """  # noqa: D205
        if "labels" not in batch:
            raise ValueError("Labels not provided in the batch. These are required for this loss computation.")

        # NOTE: token_logits is [sequence, batch] but labels and other fiels, including the loss are [batch, sequence]
        unreduced_token_loss = unreduced_token_loss_fn(forward_out["token_logits"], batch["labels"])  # [b s]

        # TODO(@jstjohn) also handle different output keys, like the sequence loss.

        # compute loss
        cp_size = parallel_state.get_context_parallel_world_size()
        if cp_size == 1:
            # reduce the loss across the micro batch per valid token
            loss_for_microbatch = masked_token_loss(unreduced_token_loss, batch["loss_mask"])
        else:
            # reduce the loss across the micro batch per valid token.
            # TODO(@jomitchell): Figure out who defines "num_valid_tokens_in_ub" in the batch and document/understand this.
            #  This has something to do with context parallel, and there is probably a megatron or nemo function that adds this and
            #  other necessary keys to the batch. Thanks!
            loss_for_microbatch = masked_token_loss_context_parallel(
                unreduced_token_loss, batch["loss_mask"], batch["num_valid_tokens_in_ub"]
            )

        # If we do not drop the last partial batch of validation, we need to do fancy reduction handling to support
        #  reducing the loss across the data parallel group.
        if self.validation_step and not self.val_drop_last:
            num_valid_tokens_in_microbatch = batch["loss_mask"].sum()
            if loss_for_microbatch.isnan():
                # TODO(@jomitchell): Add a unit test for this. This is the case where there are no valid tokens in the microbatch for the loss
                #  to be computed over, so we expect a NaN loss (divide by zero for a mean) but we make this an expected and non-breaking case,
                #  re-defining it as a 0 loss. This is standard in NeMo/NeMo2.
                if batch["loss_mask"].count_nonzero() != 0:
                    raise ValueError("Got NaN loss with non-empty input")
                loss_sum_for_microbatch = torch.zeros_like(num_valid_tokens_in_microbatch)
            else:
                loss_sum_for_microbatch = (
                    num_valid_tokens_in_microbatch * loss_for_microbatch
                )  # sum over all valid tokens

            # In this case we need to store the loss sum as well as the number of valid tokens in the microbatch.
            loss_sum_and_microbatch_size_all_gpu = torch.cat(
                [
                    loss_sum_for_microbatch.clone().detach().view(1),
                    Tensor([num_valid_tokens_in_microbatch]).cuda().clone().detach(),
                ]
            )
            torch.distributed.all_reduce(
                loss_sum_and_microbatch_size_all_gpu,
                group=parallel_state.get_data_parallel_group(),
                op=torch.distributed.ReduceOp.SUM,
            )
            return loss_for_microbatch * cp_size, {
                "loss_sum_and_microbatch_size": loss_sum_and_microbatch_size_all_gpu
            }

        # average the losses across the data parallel group, but also return the unreduced loss
        reduced_loss = average_losses_across_data_parallel_group([loss_for_microbatch])
        return loss_for_microbatch * cp_size, {"avg": reduced_loss}

__init__(validation_step=False, val_drop_last=True)

Initializes the Model class.

Parameters:

Name	Type	Description	Default
validation_step	bool	Whether this object is being applied to the validation step. Defaults to False.	False
val_drop_last	bool	Whether the last batch is configured to be dropped during validation. Defaults to True.	True

Source code in bionemo/llm/model/loss.py

def __init__(
    self,
    validation_step: bool = False,
    val_drop_last: bool = True,
) -> None:
    """Initializes the Model class.

    Args:
        validation_step (bool, optional): Whether this object is being applied to the validation step. Defaults to False.
        val_drop_last (bool, optional): Whether the last batch is configured to be dropped during validation. Defaults to True.
    """
    # TODO(@jomitchell): Track down how we handle test. This is a common pattern in NeMo2, but these parameters seem likely
    #  to change in the future.
    super().__init__()
    self.validation_step = validation_step
    self.val_drop_last = val_drop_last

forward(batch, forward_out)

Computes loss of labels in the batch vs token_logits in the forward output currently. In the future this will be extended to handle other loss types like sequence loss if it is present in the forward_out and batch.

Parameters:

Name	Type	Description	Default
batch	Dict[str, Tensor]	The batch of data. Each tensor should be of shape [batch_size, , ], and match the corresponding dimension for that particular key in the batch output. For example, the "labels" and "token_logits" key should have a tensor of shape [batch_size, sequence_length].	required
forward_out	Dict[str, Tensor]	The forward output from the model. Each tensor should be of shape [batch_size, , ]	required

Taken from: https://github.com/NVIDIA/NeMo/blob/main/nemo/collections/nlp/models/language_modeling/megatron_gpt_model.py#L951-L976 .

Source code in bionemo/llm/model/loss.py

def forward(
    self, batch: Dict[str, Tensor], forward_out: Dict[str, Tensor]
) -> Tuple[Tensor, PerTokenLossDict | SameSizeLossDict | DataParallelGroupLossAndIO]:
    """Computes loss of `labels` in the batch vs `token_logits` in the forward output currently. In the future this will be extended
        to handle other loss types like sequence loss if it is present in the forward_out and batch.

    Args:
        batch (Dict[str, Tensor]): The batch of data. Each tensor should be of shape [batch_size, *, *],
            and match the corresponding dimension for that particular key in the batch output.
            For example, the "labels" and "token_logits" key should have a tensor of shape [batch_size, sequence_length].
        forward_out (Dict[str, Tensor]): The forward output from the model. Each tensor should be of shape [batch_size, *, *]

    Taken from:
    https://github.com/NVIDIA/NeMo/blob/main/nemo/collections/nlp/models/language_modeling/megatron_gpt_model.py#L951-L976 .
    """  # noqa: D205
    if "labels" not in batch:
        raise ValueError("Labels not provided in the batch. These are required for this loss computation.")

    # NOTE: token_logits is [sequence, batch] but labels and other fiels, including the loss are [batch, sequence]
    unreduced_token_loss = unreduced_token_loss_fn(forward_out["token_logits"], batch["labels"])  # [b s]

    # TODO(@jstjohn) also handle different output keys, like the sequence loss.

    # compute loss
    cp_size = parallel_state.get_context_parallel_world_size()
    if cp_size == 1:
        # reduce the loss across the micro batch per valid token
        loss_for_microbatch = masked_token_loss(unreduced_token_loss, batch["loss_mask"])
    else:
        # reduce the loss across the micro batch per valid token.
        # TODO(@jomitchell): Figure out who defines "num_valid_tokens_in_ub" in the batch and document/understand this.
        #  This has something to do with context parallel, and there is probably a megatron or nemo function that adds this and
        #  other necessary keys to the batch. Thanks!
        loss_for_microbatch = masked_token_loss_context_parallel(
            unreduced_token_loss, batch["loss_mask"], batch["num_valid_tokens_in_ub"]
        )

    # If we do not drop the last partial batch of validation, we need to do fancy reduction handling to support
    #  reducing the loss across the data parallel group.
    if self.validation_step and not self.val_drop_last:
        num_valid_tokens_in_microbatch = batch["loss_mask"].sum()
        if loss_for_microbatch.isnan():
            # TODO(@jomitchell): Add a unit test for this. This is the case where there are no valid tokens in the microbatch for the loss
            #  to be computed over, so we expect a NaN loss (divide by zero for a mean) but we make this an expected and non-breaking case,
            #  re-defining it as a 0 loss. This is standard in NeMo/NeMo2.
            if batch["loss_mask"].count_nonzero() != 0:
                raise ValueError("Got NaN loss with non-empty input")
            loss_sum_for_microbatch = torch.zeros_like(num_valid_tokens_in_microbatch)
        else:
            loss_sum_for_microbatch = (
                num_valid_tokens_in_microbatch * loss_for_microbatch
            )  # sum over all valid tokens

        # In this case we need to store the loss sum as well as the number of valid tokens in the microbatch.
        loss_sum_and_microbatch_size_all_gpu = torch.cat(
            [
                loss_sum_for_microbatch.clone().detach().view(1),
                Tensor([num_valid_tokens_in_microbatch]).cuda().clone().detach(),
            ]
        )
        torch.distributed.all_reduce(
            loss_sum_and_microbatch_size_all_gpu,
            group=parallel_state.get_data_parallel_group(),
            op=torch.distributed.ReduceOp.SUM,
        )
        return loss_for_microbatch * cp_size, {
            "loss_sum_and_microbatch_size": loss_sum_and_microbatch_size_all_gpu
        }

    # average the losses across the data parallel group, but also return the unreduced loss
    reduced_loss = average_losses_across_data_parallel_group([loss_for_microbatch])
    return loss_for_microbatch * cp_size, {"avg": reduced_loss}

DataParallelGroupLossAndIO

Bases: TypedDict

Average losses across the data parallel group + the original batch and inference output.

Source code in bionemo/llm/model/loss.py

class DataParallelGroupLossAndIO(TypedDict):
    """Average losses across the data parallel group + the original batch and inference output."""

    avg: Tensor
    batch: dict[str, Tensor]
    forward_out: dict[str, Tensor]

PerTokenLossDict

Bases: TypedDict

Tensor dictionary for loss.

This is the return type for a loss that is computed per token in the batch, supporting microbatches of varying sizes.

Source code in bionemo/llm/model/loss.py

class PerTokenLossDict(TypedDict):
    """Tensor dictionary for loss.

    This is the return type for a loss that is computed per token in the batch, supporting microbatches of varying sizes.
    """

    loss_sum_and_microbatch_size: Tensor

SameSizeLossDict

Bases: TypedDict

Tensor dictionary for loss.

This is the return type for a loss that is computed for the entire batch, where all microbatches are the same size.

Source code in bionemo/llm/model/loss.py

class SameSizeLossDict(TypedDict):
    """Tensor dictionary for loss.

    This is the return type for a loss that is computed for the entire batch, where all microbatches are the same size.
    """

    avg: Tensor

unreduced_token_loss_fn(logits, labels, cross_entropy_loss_fusion=False)

Computes the unreduced token loss given the logits and labels without regard to the loss mask.

WARNING: This function does not apply a loss mask. Also, it does inplace operation on the inputs.

Parameters:

Name	Type	Description	Default
logits	Tensor	The predicted logits of shape [sequence_length, batch_size, num_classes].	required
labels	Tensor	The true labels of shape [batch_size, sequence_length].	required
cross_entropy_loss_fusion	bool	If True, use the fused kernel version of vocab parallel cross entropy. This should generally be preferred for speed as it packs more operations into a single kernel on the GPU. However some users have observed reduced training stability when using this method.	False

Returns:

Name	Type	Description
Tensor	Tensor	The unreduced token loss of shape [batch_size, sequence_length].

Source code in bionemo/llm/model/loss.py

def unreduced_token_loss_fn(logits: Tensor, labels: Tensor, cross_entropy_loss_fusion: bool = False) -> Tensor:
    """Computes the unreduced token loss given the logits and labels without regard to the loss mask.

    WARNING: This function does not apply a loss mask. Also, it does inplace operation on the inputs.

    Args:
        logits (Tensor): The predicted logits of shape [sequence_length, batch_size, num_classes].
        labels (Tensor): The true labels of shape [batch_size, sequence_length].
        cross_entropy_loss_fusion (bool): If True, use the fused kernel version of vocab parallel cross entropy. This
            should generally be preferred for speed as it packs more operations into a single kernel on the GPU. However
            some users have observed reduced training stability when using this method.

    Returns:
        Tensor: The unreduced token loss of shape [batch_size, sequence_length].
    """
    labels = labels.transpose(0, 1).contiguous()  # [b, s] -> [s, b]
    if cross_entropy_loss_fusion:
        loss = fused_vocab_parallel_cross_entropy(logits, labels)
    else:
        loss = tensor_parallel.vocab_parallel_cross_entropy(logits, labels)
    # [s b] => [b, s]
    loss = loss.transpose(0, 1).contiguous()
    return loss