mmpretrain/models/selfsup/mae.py

# Copyright (c) OpenMMLab. All rights reserved.
from typing import Dict, List, Optional, Sequence, Tuple, Union

import torch

from mmpretrain.models import VisionTransformer
from mmpretrain.registry import MODELS
from mmpretrain.structures import DataSample
from ..utils import build_2d_sincos_position_embedding
from .base import BaseSelfSupervisor


@MODELS.register_module()
class MAEViT(VisionTransformer):
    """Vision Transformer for MAE pre-training.

    A PyTorch implement of: `An Image is Worth 16x16 Words: Transformers
    for Image Recognition at Scale <https://arxiv.org/abs/2010.11929>`_.
    This module implements the patch masking in MAE and initialize the
    position embedding with sine-cosine position embedding.

    Args:
        arch (str | dict): Vision Transformer architecture
            Default: 'b'
        img_size (int | tuple): Input image size
        patch_size (int | tuple): The patch size
        out_indices (Sequence | int): Output from which stages.
            Defaults to -1, means the last stage.
        drop_rate (float): Probability of an element to be zeroed.
            Defaults to 0.
        drop_path_rate (float): stochastic depth rate. Defaults to 0.
        norm_cfg (dict): Config dict for normalization layer.
            Defaults to ``dict(type='LN')``.
        final_norm (bool): Whether to add a additional layer to normalize
            final feature map. Defaults to True.
        out_type (str): The type of output features. Please choose from

            - ``"cls_token"``: The class token tensor with shape (B, C).
            - ``"featmap"``: The feature map tensor from the patch tokens
              with shape (B, C, H, W).
            - ``"avg_featmap"``: The global averaged feature map tensor
              with shape (B, C).
            - ``"raw"``: The raw feature tensor includes patch tokens and
              class tokens with shape (B, L, C).

            It only works without input mask. Defaults to ``"avg_featmap"``.
        interpolate_mode (str): Select the interpolate mode for position
            embeding vector resize. Defaults to "bicubic".
        patch_cfg (dict): Configs of patch embeding. Defaults to an empty dict.
        layer_cfgs (Sequence | dict): Configs of each transformer layer in
            encoder. Defaults to an empty dict.
        mask_ratio (bool): The ratio of total number of patches to be masked.
            Defaults to 0.75.
        init_cfg (Union[List[dict], dict], optional): Initialization config
            dict. Defaults to None.
    """

    def __init__(self,
                 arch: Union[str, dict] = 'b',
                 img_size: int = 224,
                 patch_size: int = 16,
                 out_indices: Union[Sequence, int] = -1,
                 drop_rate: float = 0,
                 drop_path_rate: float = 0,
                 norm_cfg: dict = dict(type='LN', eps=1e-6),
                 final_norm: bool = True,
                 out_type: str = 'raw',
                 interpolate_mode: str = 'bicubic',
                 patch_cfg: dict = dict(),
                 layer_cfgs: dict = dict(),
                 mask_ratio: float = 0.75,
                 init_cfg: Optional[Union[List[dict], dict]] = None) -> None:
        super().__init__(
            arch=arch,
            img_size=img_size,
            patch_size=patch_size,
            out_indices=out_indices,
            drop_rate=drop_rate,
            drop_path_rate=drop_path_rate,
            norm_cfg=norm_cfg,
            final_norm=final_norm,
            out_type=out_type,
            with_cls_token=True,
            interpolate_mode=interpolate_mode,
            patch_cfg=patch_cfg,
            layer_cfgs=layer_cfgs,
            init_cfg=init_cfg)

        # position embedding is not learnable during pretraining
        self.pos_embed.requires_grad = False
        self.mask_ratio = mask_ratio
        self.num_patches = self.patch_resolution[0] * self.patch_resolution[1]

    def init_weights(self) -> None:
        """Initialize position embedding, patch embedding and cls token."""
        super().init_weights()
        pos_embed = build_2d_sincos_position_embedding(
            int(self.num_patches**.5),
            self.pos_embed.shape[-1],
            cls_token=True)
        self.pos_embed.data.copy_(pos_embed.float())

        w = self.patch_embed.projection.weight.data
        torch.nn.init.xavier_uniform_(w.view([w.shape[0], -1]))

        torch.nn.init.normal_(self.cls_token, std=.02)

    def random_masking(
        self,
        x: torch.Tensor,
        mask_ratio: float = 0.75
    ) -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor]:
        """Generate the mask for MAE Pre-training.

        Args:
            x (torch.Tensor): Image with data augmentation applied, which is
                of shape B x L x C.
            mask_ratio (float): The mask ratio of total patches.
                Defaults to 0.75.

        Returns:
            Tuple[torch.Tensor, torch.Tensor, torch.Tensor]: masked image, mask
            and the ids to restore original image.

            - ``x_masked`` (torch.Tensor): masked image.
            - ``mask`` (torch.Tensor): mask used to mask image.
            - ``ids_restore`` (torch.Tensor): ids to restore original image.
        """
        N, L, D = x.shape  # batch, length, dim
        len_keep = int(L * (1 - mask_ratio))

        noise = torch.rand(N, L, device=x.device)  # noise in [0, 1]

        # sort noise for each sample
        ids_shuffle = torch.argsort(
            noise, dim=1)  # ascend: small is keep, large is remove
        ids_restore = torch.argsort(ids_shuffle, dim=1)

        # keep the first subset
        ids_keep = ids_shuffle[:, :len_keep]
        x_masked = torch.gather(
            x, dim=1, index=ids_keep.unsqueeze(-1).repeat(1, 1, D))

        # generate the binary mask: 0 is keep, 1 is remove
        mask = torch.ones([N, L], device=x.device)
        mask[:, :len_keep] = 0
        # unshuffle to get the binary mask
        mask = torch.gather(mask, dim=1, index=ids_restore)

        return x_masked, mask, ids_restore

    def forward(
        self,
        x: torch.Tensor,
        mask: Optional[bool] = True
    ) -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor]:
        """Generate features for masked images.

        The function supports two kind of forward behaviors. If the ``mask`` is
        ``True``, the function will generate mask to masking some patches
        randomly and get the hidden features for visible patches, which means
        the function will be executed as masked imagemodeling pre-training;
        if the ``mask`` is ``None`` or ``False``, the forward function will
        call ``super().forward()``, which extract features from images without
        mask.


        Args:
            x (torch.Tensor): Input images, which is of shape B x C x H x W.
            mask (bool, optional): To indicate whether the forward function
                generating ``mask`` or not.

        Returns:
            Tuple[torch.Tensor, torch.Tensor, torch.Tensor]: Hidden features,
            mask and the ids to restore original image.

            - ``x`` (torch.Tensor): hidden features, which is of shape
              B x (L * mask_ratio) x C.
            - ``mask`` (torch.Tensor): mask used to mask image.
            - ``ids_restore`` (torch.Tensor): ids to restore original image.
        """
        if mask is None or False:
            return super().forward(x)

        else:
            B = x.shape[0]
            x = self.patch_embed(x)[0]
            # add pos embed w/o cls token
            x = x + self.pos_embed[:, 1:, :]

            # masking: length -> length * mask_ratio
            x, mask, ids_restore = self.random_masking(x, self.mask_ratio)

            # append cls token
            cls_token = self.cls_token + self.pos_embed[:, :1, :]
            cls_tokens = cls_token.expand(B, -1, -1)
            x = torch.cat((cls_tokens, x), dim=1)

            for _, layer in enumerate(self.layers):
                x = layer(x)
            # Use final norm
            x = self.norm1(x)

            return (x, mask, ids_restore)


@MODELS.register_module()
class MAE(BaseSelfSupervisor):
    """MAE.

    Implementation of `Masked Autoencoders Are Scalable Vision Learners
    <https://arxiv.org/abs/2111.06377>`_.
    """

    def extract_feat(self, inputs: torch.Tensor):
        return self.backbone(inputs, mask=None)

    def loss(self, inputs: torch.Tensor, data_samples: List[DataSample],
             **kwargs) -> Dict[str, torch.Tensor]:
        """The forward function in training.

        Args:
            inputs (torch.Tensor): The input images.
            data_samples (List[DataSample]): All elements required
                during the forward function.

        Returns:
            Dict[str, torch.Tensor]: A dictionary of loss components.
        """
        # ids_restore: the same as that in original repo, which is used
        # to recover the original order of tokens in decoder.
        latent, mask, ids_restore = self.backbone(inputs)
        pred = self.neck(latent, ids_restore)
        loss = self.head.loss(pred, inputs, mask)
        losses = dict(loss=loss)
        return losses