gpt_dataset.py

# Copyright (c) 2023, NVIDIA CORPORATION. All rights reserved.

import logging
import os
import time
from dataclasses import dataclass
from typing import Dict, Optional, Tuple

import numpy
import torch

from megatron.core.datasets.blended_megatron_dataset_config import BlendedMegatronDatasetConfig
from megatron.core.datasets.indexed_dataset import IndexedDataset
from megatron.core.datasets.megatron_dataset import MegatronDataset
from megatron.core.datasets.megatron_tokenizer import MegatronTokenizer
from megatron.core.datasets.utils import Split
from megatron.core.datasets.utils_s3 import S3Config, is_s3_path
from megatron.core.utils import log_single_rank

logger = logging.getLogger(__name__)

_PAD_TOKEN_ID = -1


@dataclass
class GPTDatasetConfig(BlendedMegatronDatasetConfig):
    """Configuration object for Megatron Core GPT datasets"""

    reset_position_ids: bool = None
    """Option to reset the position IDs in the dataset at an interval"""

    reset_attention_mask: bool = None
    """Option to reset the attention mask from the dataset"""

    eod_mask_loss: bool = None
    """Option to enable the EOD mask loss"""

    create_attention_mask: bool = True
    """Option to enable the attention masks generation. Can be disabled if attention kernel
       generates masks by itself.
    """

    drop_last_partial_validation_sequence: bool = True
    """Option to drop the last partial validation sequence"""

    add_extra_token_to_sequence: bool = True
    """Option to draw sequences with one extra token to ensure the sample input tokens and sample
       output tokens are both of the desired sequence length
    """

    s3_cache_path: str = None
    """Path for caching indices for s3 dataloading."""

    def __post_init__(self) -> None:
        """Do asserts and set fields post init"""
        super().__post_init__()

        assert self.tokenizer is not None

        assert self.reset_position_ids is not None
        assert self.reset_attention_mask is not None
        assert self.eod_mask_loss is not None


class GPTDataset(MegatronDataset):
    """The base GPT dataset

    Args:
        indexed_dataset (IndexedDataset): The IndexedDataset around which to build the GPTDataset

        dataset_path (Optional[str]): The real path on disk to the dataset, for bookkeeping

        indexed_indices (numpy.ndarray): The set of the documents indices to expose

        num_samples (Optional[int]): The number of samples to draw from the indexed dataset. When
            None, build as many samples as correspond to one epoch.

        index_split (Split): The indexed_indices Split

        config (GPTDatasetConfig): The config
    """

    def __init__(
        self,
        indexed_dataset: IndexedDataset,
        dataset_path: Optional[str],
        indexed_indices: numpy.ndarray,
        num_samples: Optional[int],
        index_split: Split,
        config: GPTDatasetConfig,
    ) -> None:
        super().__init__(
            indexed_dataset, dataset_path, indexed_indices, num_samples, index_split, config
        )
        self.masks_and_position_ids_are_cacheable = not any(
            [
                self.config.reset_position_ids,
                self.config.reset_attention_mask,
                self.config.eod_mask_loss,
            ]
        )
        self.masks_and_position_ids_are_cached = False
        self.cached_attention_mask = None
        self.cached_loss_mask = None
        self.cached_position_ids = None

        try:
            self._pad_token_id = self.config.tokenizer.pad
        except Exception:
            self._pad_token_id = _PAD_TOKEN_ID

        (self.document_index, self.sample_index, self.shuffle_index) = (
            self._build_document_sample_shuffle_indices()
        )

    @staticmethod
    def numel_low_level_dataset(low_level_dataset: IndexedDataset) -> int:
        """Abstract method implementation

        For GPT, the underlying IndexedDataset should be split by sequence, as opposed to, say,
        BERT, which should be split by document

        Args:
            low_level_dataset (IndexedDataset): The underlying IndexedDataset

        Returns:
            int: The number of unique elements in the underlying IndexedDataset
        """
        return low_level_dataset.sequence_lengths.shape[0]

    @staticmethod
    def build_low_level_dataset(dataset_path: str, config: GPTDatasetConfig) -> IndexedDataset:
        """Abstract method implementation

        Args:
            dataset_path (str): The real path prefix to the IndexedDataset .bin and .idx files

            config (GPTDatasetConfig): The config

        Returns:
            IndexedDataset: The underlying IndexedDataset
        """
        if is_s3_path(dataset_path):
            return IndexedDataset(
                dataset_path,
                multimodal=False,
                mmap=config.mmap_bin_files,
                s3_config=S3Config(path_to_idx_cache=config.s3_cache_path),
            )
        return IndexedDataset(dataset_path, multimodal=False, mmap=config.mmap_bin_files)

    def __len__(self) -> int:
        """Abstract method implementation

        Returns:
            int: The length of the dataset
        """
        return self.sample_index.shape[0] - 1

    def __getitem__(self, idx: Optional[int]) -> Dict[str, torch.Tensor]:
        """Abstract method implementation

        Args:
            idx (Optioal[int]): The index into the dataset

        Returns:
            Dict[str, torch.Tensor]: The sample information wrapped in a dictionary
        """
        if idx is None:
            # Batch padding sequence so the index does not matter
            text, _ = self._query_document_sample_shuffle_indices(0)
        else:
            text, _ = self._query_document_sample_shuffle_indices(idx)

        text = torch.from_numpy(text).long()
        if self.config.add_extra_token_to_sequence:
            tokens = text[:-1].contiguous()
            labels = text[1:].contiguous()
        else:
            tokens = text
            labels = torch.roll(text, shifts=-1, dims=0)
            labels[-1] = self._pad_token_id

        if (
            not self.masks_and_position_ids_are_cacheable
            or not self.masks_and_position_ids_are_cached
        ):
            attention_mask, loss_mask, position_ids = _get_ltor_masks_and_position_ids(
                tokens,
                self.config.tokenizer.eod,
                self.config.reset_position_ids,
                self.config.reset_attention_mask,
                self.config.eod_mask_loss,
                self.config.create_attention_mask,
            )
            if self.masks_and_position_ids_are_cacheable:
                self.cached_attention_mask = attention_mask
                self.cached_loss_mask = loss_mask
                self.cached_position_ids = position_ids
                self.masks_and_position_ids_are_cached = True
        else:
            attention_mask = self.cached_attention_mask
            loss_mask = self.cached_loss_mask
            position_ids = self.cached_position_ids

        # For padded sequences, mask the loss
        loss_mask[labels == self._pad_token_id] = 0.0

        # For padded sequences, ensure the embedding layer can map the token ID
        tokens[tokens == self._pad_token_id] = 0
        labels[labels == self._pad_token_id] = 0

        # Batch padding sequence so we mask the loss
        if idx is None:
            loss_mask = torch.zeros_like(loss_mask)

        if self.config.create_attention_mask:
            return {
                "tokens": tokens,
                "labels": labels,
                "attention_mask": attention_mask,
                "loss_mask": loss_mask,
                "position_ids": position_ids,
            }
        else:
            return {
                "tokens": tokens,
                "labels": labels,
                "loss_mask": loss_mask,
                "position_ids": position_ids,
            }

    def _query_document_sample_shuffle_indices(
        self, idx: int
    ) -> Tuple[numpy.ndarray, numpy.ndarray]:
        """Get the text (token ids) and document ids for a given index

        Args:
            idx (int): The index into the dataset

        Returns:
            Tuple[numpy.ndarray, numpy.ndarray]: The text ids and document ids
        """
        # Do the shuffle mapping
        idx = self.shuffle_index[idx]

        # Get the beginning and end documents and offsets
        doc_index_beg, doc_index_beg_offset = self.sample_index[idx]
        doc_index_end, doc_index_end_offset = self.sample_index[idx + 1]

        document_ids = []
        sample_parts = []

        # Sample spans a single document
        if doc_index_beg == doc_index_end:
            # Add the document id
            document_ids.append(self.document_index[doc_index_beg])

            # Add the entire sample
            sample_parts.append(
                self.dataset.get(
                    self.document_index[doc_index_beg],
                    offset=doc_index_beg_offset,
                    length=doc_index_end_offset
                    - doc_index_beg_offset
                    + self.config.add_extra_token_to_sequence,
                )
            )

        # Sample spans multiple documents
        else:
            for i in range(doc_index_beg, doc_index_end + 1):
                # Add the document id
                document_ids.append(self.document_index[i])

                # Add the sample part
                offset = 0 if i > doc_index_beg else doc_index_beg_offset
                length = (
                    None
                    if i < doc_index_end
                    else doc_index_end_offset + self.config.add_extra_token_to_sequence
                )
                sample_parts.append(
                    self.dataset.get(self.document_index[i], offset=offset, length=length)
                )
        assert len(document_ids) == len(
            sample_parts
        ), f"len(document_ids) ({len(document_ids)}) != len(sample_parts) ({len(sample_parts)})"

        length = sum(map(len, sample_parts))

        # Pad the sample if necessary
        if length < (self.config.sequence_length + self.config.add_extra_token_to_sequence):
            sample_parts.append(
                [self._pad_token_id]
                * (self.config.sequence_length + self.config.add_extra_token_to_sequence - length)
            )

        return (
            numpy.concatenate(sample_parts, dtype=numpy.int64),
            numpy.array(document_ids, dtype=numpy.int64),
        )

    def _build_document_sample_shuffle_indices(
        self,
    ) -> Tuple[numpy.ndarray, numpy.ndarray, numpy.ndarray]:
        """Build the document index, the sample index, and the shuffle index

        The document index:
            -- 1-D
            -- An ordered array of document ids

        The sample index:
            -- 2-D
            -- The document indices and offsets which mark the start of every sample

        The shuffle index:
            -- 1-D
            -- A random permutation of index range of the sample index

        Returns:
            Tuple[numpy.ndarray, numpy.ndarray, numpy.ndarray]: The document index, the sample
            index, and the shuffle index
        """
        path_to_cache = self.config.path_to_cache
        if path_to_cache is None and not self.config.mock:
            path_to_cache = os.path.join(
                self.dataset.path_prefix, "cache", f"{type(self).__name__}_indices"
            )

        if path_to_cache:
            base = f"{self.unique_description_hash}-{type(self).__name__}-{self.index_split.name}"
            get_path_to = lambda affix: os.path.join(path_to_cache, f"{base}-{affix}")
            path_to_description = get_path_to("description.txt")
            path_to_document_index = get_path_to("document_index.npy")
            path_to_sample_index = get_path_to("sample_index.npy")
            path_to_shuffle_index = get_path_to("shuffle_index.npy")
            cache_hit = all(
                map(
                    os.path.isfile,
                    [
                        path_to_description,
                        path_to_document_index,
                        path_to_sample_index,
                        path_to_shuffle_index,
                    ],
                )
            )
        else:
            cache_hit = False

        if not path_to_cache or (
            not cache_hit
            and (not torch.distributed.is_initialized() or torch.distributed.get_rank() == 0)
        ):

            log_single_rank(
                logger,
                logging.INFO,
                f"Build and save the {type(self).__name__} {self.index_split.name} indices",
            )
            self.built_anew_on_cache_miss = True
            t_beg = time.time()

            sequence_length = self.config.sequence_length
            num_tokens_per_epoch = self._get_num_tokens_per_epoch()
            num_epochs = self._get_num_epochs(num_tokens_per_epoch)

            if num_epochs == 1:
                separate_final_epoch = False
            else:
                # Get the number of samples for the last epoch
                num_samples_sans_final_epoch = (
                    (num_epochs - 1) * num_tokens_per_epoch
                    - self.config.add_extra_token_to_sequence
                ) // sequence_length
                num_samples_from_final_epoch = self.num_samples - num_samples_sans_final_epoch
                num_samples_per_epoch = (
                    num_tokens_per_epoch - self.config.add_extra_token_to_sequence
                ) // sequence_length

                # num_samples_from_final_epoch should be non-negative
                assert num_samples_from_final_epoch >= 0

                # num_samples_from_final_epoch should not exceed max value
                assert num_samples_from_final_epoch <= num_samples_per_epoch + 1

                # Separate the final epoch if it falls below the threshold
                threshold = 0.80
                separate_final_epoch = num_samples_from_final_epoch < int(
                    threshold * num_samples_per_epoch
                )

                log_single_rank(
                    logger,
                    logging.DEBUG,
                    f"> num_samples_from_final_epoch: {num_samples_from_final_epoch}",
                )
                log_single_rank(logger, logging.DEBUG, f"> threshold: {threshold}")
                log_single_rank(
                    logger, logging.DEBUG, f"> num_samples_per_epoch: {num_samples_per_epoch}"
                )

            log_single_rank(
                logger, logging.DEBUG, f"> separate_final_epoch: {separate_final_epoch}"
            )

            numpy_random_state = numpy.random.RandomState(self.config.random_seed)

            # Build the document index
            document_index = _build_document_index(
                self.indices, num_epochs, numpy_random_state, separate_final_epoch
            )

            drop_last_partial_sequence = True
            if self.index_split == Split.valid:
                drop_last_partial_sequence = self.config.drop_last_partial_validation_sequence

            # Build the sample index
            from megatron.core.datasets import helpers

            if self.index_split == Split.valid:
                drop_last_partial_sequence = self.config.drop_last_partial_validation_sequence
            else:
                drop_last_partial_sequence = True

            assert document_index.dtype == numpy.int32
            assert self.dataset.sequence_lengths.dtype == numpy.int32
            if len(document_index) * 2 > len(self.dataset.sequence_lengths):
                # If "access density" of sequence_lengths is high, force load the mmap-ed array
                # into memory by making a copy.
                #
                # System performance benefits come from two aspects:
                #   1. We sequentially pre-load the whole file, most of which we expect to read
                #   2. The GIL is held when entering the c++ program, improving the speed of which
                #      improves parallelism
                sequence_lengths_for_cpp = self.dataset.sequence_lengths.copy()
            else:
                sequence_lengths_for_cpp = self.dataset.sequence_lengths
            sample_index = helpers.build_sample_idx(
                sequence_lengths_for_cpp,
                document_index,
                sequence_length,
                num_epochs,
                num_tokens_per_epoch,
                drop_last_partial_sequence,
                self.config.add_extra_token_to_sequence,
            )

            # Build the shuffle index
            if separate_final_epoch:
                shuffle_index = _build_shuffle_index(
                    num_samples_sans_final_epoch, sample_index.shape[0] - 1, numpy_random_state
                )
            else:
                shuffle_index = _build_shuffle_index(
                    sample_index.shape[0] - 1, sample_index.shape[0] - 1, numpy_random_state
                )

            if path_to_cache:
                os.makedirs(path_to_cache, exist_ok=True)
                # Write the description
                with open(path_to_description, "wt") as writer:
                    writer.write(self.unique_description)
                numpy.save(path_to_document_index, document_index, allow_pickle=True)
                numpy.save(path_to_sample_index, sample_index, allow_pickle=True)
                numpy.save(path_to_shuffle_index, shuffle_index, allow_pickle=True)
            else:
                log_single_rank(
                    logger,
                    logging.WARNING,
                    f"Unable to save {type(self).__name__} indexes because path_to_cache is None",
                )

            t_end = time.time()
            log_single_rank(logger, logging.DEBUG, f"\t> time elapsed: {t_end - t_beg:4f} seconds")

            log_single_rank(
                logger, logging.INFO, f"> total number of samples: {sample_index.shape[0] - 1}"
            )
            log_single_rank(logger, logging.INFO, f"> total number of epochs: {num_epochs}")

            return document_index, sample_index, shuffle_index

        log_single_rank(
            logger, logging.INFO, f"Load the {type(self).__name__} {self.index_split.name} indices"
        )

        log_single_rank(
            logger,
            logging.INFO,
            f"\tLoad the document index from {os.path.basename(path_to_document_index)}",
        )
        t_beg = time.time()
        document_index = numpy.load(path_to_document_index, allow_pickle=True, mmap_mode='r')
        t_end = time.time()
        log_single_rank(logger, logging.DEBUG, f"\t> time elapsed: {t_end - t_beg:4f} seconds")

        log_single_rank(
            logger,
            logging.INFO,
            f"\tLoad the sample index from {os.path.basename(path_to_sample_index)}",
        )
        t_beg = time.time()
        sample_index = numpy.load(path_to_sample_index, allow_pickle=True, mmap_mode='r')
        t_end = time.time()
        log_single_rank(logger, logging.DEBUG, f"\t> time elapsed: {t_end - t_beg:4f} seconds")

        log_single_rank(
            logger,
            logging.INFO,
            f"\tLoad the shuffle index from {os.path.basename(path_to_shuffle_index)}",
        )
        t_beg = time.time()
        shuffle_index = numpy.load(path_to_shuffle_index, allow_pickle=True, mmap_mode='r')
        t_end = time.time()
        log_single_rank(logger, logging.DEBUG, f"\t> time elapsed: {t_end - t_beg:4f} seconds")

        log_single_rank(
            logger, logging.INFO, f"> total number of samples: {sample_index.shape[0] - 1}"
        )

        return document_index, sample_index, shuffle_index

    def _get_num_tokens_per_epoch(self) -> int:
        """Calculate the number of tokens in a single epoch

        Returns:
            int: The number of tokens in a single epoch
        """
        return int(numpy.sum(self.dataset.sequence_lengths[self.indices]))

    def _get_num_epochs(self, num_tokens_per_epoch: int) -> int:
        """Calculate the number of epochs

        Args:
            num_tokens_per_epoch (int): The number of tokens in a single epoch

        Returns:
            int: The number of epochs
        """
        num_epochs = 1
        num_tokens = num_tokens_per_epoch
        if self.num_samples is None:
            return num_epochs
        else:
            num_tokens_requested = (
                self.num_samples * self.config.sequence_length
            ) + self.config.add_extra_token_to_sequence
            while num_tokens < num_tokens_requested:
                num_epochs += 1
                num_tokens += num_tokens_per_epoch
        return num_epochs


def _build_document_index(
    documents: numpy.ndarray,
    num_epochs: int,
    numpy_random_state: numpy.random.RandomState,
    separate_final_epoch: bool,
) -> numpy.ndarray:
    """Build an array with length = num epochs * num documents

    Args:
        documents (numpy.ndarray): the subset of exposed document indices

        num_epochs (int): The number of epochs

        numpy_random_state (numpy.random.RandomState): The NumPy random state

        separate_final_epoch (bool): Whether to exclude the last epoch from the global shuffle

    Returns:
        numpy.ndarray: The document index
    """
    if not separate_final_epoch or num_epochs == 1:
        document_index = numpy.mgrid[0:num_epochs, 0 : len(documents)][1]
        document_index[:] = documents
        document_index = document_index.reshape(-1)
        document_index = document_index.astype(numpy.int32)
        numpy_random_state.shuffle(document_index)
        return document_index

    doc_idx_first = _build_document_index(documents, num_epochs - 1, numpy_random_state, False)
    doc_idx_last = _build_document_index(documents, 1, numpy_random_state, False)
    return numpy.concatenate((doc_idx_first, doc_idx_last))


def _build_shuffle_index(
    num_samples: int, total_size: int, numpy_random_state: numpy.random.RandomState
) -> numpy.ndarray:
    """Build the range [0, size) and shuffle

    Args:
        num_samples (int): The size of the first shuffle range [0, num_samples)

        total_size (int): The size of the entire index. If larger than 'num_samples', it defines
            the second shuffle range [num_samples, total_size)

        numpy_random_state (numpy.random.RandomState): The NumPy random state

    Returns:
        numpy.ndarray: The shuffle index
    """
    dtype_ = numpy.uint32
    if total_size >= (numpy.iinfo(numpy.uint32).max - 1):
        dtype_ = numpy.int64

    shuffle_idx_first = numpy.arange(start=0, stop=num_samples, step=1, dtype=dtype_)
    numpy_random_state.shuffle(shuffle_idx_first)
    if num_samples == total_size:
        return shuffle_idx_first

    shuffle_idx_last = numpy.arange(start=num_samples, stop=total_size, step=1, dtype=dtype_)
    numpy_random_state.shuffle(shuffle_idx_last)

    return numpy.concatenate((shuffle_idx_first, shuffle_idx_last))


def _get_ltor_masks_and_position_ids(
    data: torch.Tensor,
    eod_token: int,
    reset_position_ids: bool,
    reset_attention_mask: bool,
    eod_mask_loss: bool,
    create_attention_mask: bool,
):
    """Build masks and position id for left to right model.

    Args:
        data (torch.Tensor): The data tenor that holds the tokens from the dataset

        eod_token (int): ID of the token to that is considered the EOD

        reset_position_ids (bool): Switch to reset the document position ID's

        reset_attention_mask (bool): Switch to reset the attention mask

        eod_mask_loss (bool): Switch to enable the EOD mask loss

        create_attention_mask (bool): Switch to enable the attention masks generation. Can be
            disabled if attention kernel generates masks by itself.

    Returns:
        torch.Tensor: Attention mask needed to be used for Attention

        torch.Tensor: The mask used for loss value during training

        torch.Tensor: The position ID's of the token
    """
    seq_length = data.numel()

    if create_attention_mask:
        attention_mask = torch.tril(
            torch.ones((seq_length, seq_length), device=data.device)
        ).unsqueeze(0)
    else:
        attention_mask = None

    # Loss mask.
    loss_mask = torch.ones(seq_length, dtype=torch.float, device=data.device)
    if eod_mask_loss:
        loss_mask[data == eod_token] = 0.0

    # Position ids.
    position_ids = torch.arange(seq_length, dtype=torch.long, device=data.device)
    # We need to clone as the ids will be modifed based on batch index.
    if reset_position_ids:
        position_ids = position_ids.clone()

    if reset_position_ids or reset_attention_mask:
        # Find indices where EOD token is.
        eod_index = position_ids[data == eod_token]
        # Detach indices from positions if going to modify positions.
        if reset_position_ids:
            eod_index = eod_index.clone()

        # Loop through EOD indices:
        prev_index = 0
        for j in range(eod_index.numel()):
            i = eod_index[j]
            # Mask attention loss.
            if reset_attention_mask and attention_mask is not None:
                attention_mask[0, (i + 1) :, : (i + 1)] = 0
            # Reset positions.
            if reset_position_ids:
                position_ids[(i + 1) :] -= i + 1 - prev_index
                prev_index = i + 1

    if attention_mask is not None:
        # Convert attention mask to binary:
        attention_mask = attention_mask < 0.5

    return attention_mask, loss_mask, position_ids


class MockGPTLowLevelDataset:
    """The mock GPT low level dataset

    This class is meant to generate tokenized data in the classic "Megatron-LM" GPT style. Notably,
    we add the end of document token to each element indexed in __getitem__

    Args:
        tokenizer (MegatronTokenizer): The tokenizer the special token information of which we use
            to augment the mock data.
    """

    seed: int = 0
    """The hard-coded random seed to use to set the NumPy RNG"""

    size: int = 100000
    """The hard-coded number of samples to generate"""

    max_sequence_length: int = 4096
    """The hard-coded max sequence length to generate"""

    def __init__(self, tokenizer: MegatronTokenizer) -> None:
        self.tokenizer = tokenizer
        rng = numpy.random.default_rng(seed=self.seed)
        self.sequence_lengths = rng.integers(
            low=1, high=self.max_sequence_length, size=self.size, dtype=numpy.int32
        )

    def __len__(self) -> int:
        return self.size

    def __getitem__(self, idx: int) -> numpy.number:
        length = self.sequence_lengths[idx]
        sample = numpy.int64(
            numpy.concatenate([numpy.arange(length - 1) + 1, [self.tokenizer.eod]])
        )
        return sample

    def get(self, idx: int, offset: int = 0, length: Optional[int] = None) -> numpy.ndarray:
        """This function is n abstraction over __getitem__ with support for slicing

        Args:
            idx (int): The index into the dataset

            offset (int): The integer token offset in the sequence

            length (Optional[int]): The number of tokens to grab from the sequence

        Returns:
            numpy.ndarray: The sequence tokens at the index
        """
        if length is None:
            length = self.sequence_lengths[idx] - offset
        return self[idx][offset : offset + length]


class MockGPTDataset(GPTDataset):
    """The mock GPT dataset

    Args:
        indexed_dataset (MockGPTLowLevelDataset): The MockGPTLowLevelDataset around which to build
            the MockGPTDataset

        dataset_path (Optional[str]): This argument is of no consequence for the MockGPTDataset

        indices (numpy.ndarray): The set of the dataset indices to expose

        num_samples (int): The number of samples to draw from the dataset

        index_split (Split): The indices Split

        config (GPTDatasetConfig): The config
    """

    def __init__(
        self,
        dataset: MockGPTLowLevelDataset,
        dataset_path: Optional[str],
        indices: numpy.ndarray,
        num_samples: int,
        index_split: Split,
        config: GPTDatasetConfig,
    ) -> None:
        assert config.mock

        super().__init__(dataset, dataset_path, indices, num_samples, index_split, config)

    @staticmethod
    def numel_low_level_dataset(low_level_dataset: MockGPTLowLevelDataset) -> int:
        """Abstract method implementation

        Args:
            low_level_dataset (MockGPTLowLevelDataset): The underlying MockGPTLowLevelDataset

        Returns:
            int: The number of unique elements in the underlying MockGPTLowLevelDataset
        """
        return len(low_level_dataset)

    @staticmethod
    def build_low_level_dataset(
        dataset_path: Optional[str], config: GPTDatasetConfig
    ) -> MockGPTLowLevelDataset:
        """Abstract method implementation

        Args:
            dataset_path (Optional[str]): This argument is of no consequence for the
                MockGPTLowLevelDataset

            config (GPTDatasetConfig): The config

        Returns:
            MockGPTLowLevelDataset: The underlying MockGPTLowLevelDataset
        """
        return MockGPTLowLevelDataset(config.tokenizer)