Adding ViT to torchvision/models

torchvision/models/__init__.py

@@ -10,6 +10,7 @@
 from .shufflenetv2 import *
 from .efficientnet import *
 from .regnet import *
+from .vision_transformer import *
 from . import detection
 from . import feature_extraction
 from . import quantization

torchvision/models/vision_transformer.py

@@ -0,0 +1,307 @@
+# Implement ViT from:
+# https://arxiv.org/abs/2010.11929
+
+# References:
+# https://github.com/google-research/vision_transformer
+# https://github.com/facebookresearch/ClassyVision/blob/main/classy_vision/models/vision_transformer.py
+
+import math
+from collections import OrderedDict
+from functools import partial
+from typing import Any
+
+import torch
+import torch.nn as nn
+from torch import Tensor
+
+__all__ = [
+    "VisionTransformer",
+    "vit_b_16",
+    "vit_b_32",
+    "vit_l_16",
+    "vit_l_32",
+]
+
+
+LayerNorm = partial(nn.LayerNorm, eps=1e-6)
+
+
+class MLPBlock(nn.Sequential):
+    """Transformer MLP block."""
+
+    def __init__(self, in_dim: int, mlp_dim: int, dropout_rate: float):
+        super().__init__()
+        self.linear_1 = nn.Linear(in_dim, mlp_dim)
+        self.act = nn.GELU()
+        self.dropout_1 = nn.Dropout(dropout_rate)
+        self.linear_2 = nn.Linear(mlp_dim, in_dim)
+        self.dropout_2 = nn.Dropout(dropout_rate)
+        self.init_weights()
+
+    def init_weights(self):
+        nn.init.xavier_uniform_(self.linear_1.weight)
+        nn.init.xavier_uniform_(self.linear_2.weight)
+        nn.init.normal_(self.linear_1.bias, std=1e-6)
+        nn.init.normal_(self.linear_2.bias, std=1e-6)
+
+
+class EncoderBlock(nn.Module):
+    """Transformer encoder block."""
+
+    def __init__(
+        self, num_heads: int, hidden_dim: int, mlp_dim: int, dropout_rate: float, attention_dropout_rate: float
+    ):
+        super().__init__()
+        self.num_heads = num_heads
+
+        # Attention block
+        self.ln_1 = LayerNorm(hidden_dim)
+        self.self_attention = nn.MultiheadAttention(hidden_dim, num_heads, dropout=attention_dropout_rate)
+        self.dropout = nn.Dropout(dropout_rate)
+
+        # MLP block
+        self.ln_2 = LayerNorm(hidden_dim)
+        self.mlp = MLPBlock(hidden_dim, mlp_dim, dropout_rate)
+
+    def forward(self, input: Tensor):
+        # assert input.dim() == 3, f"Expected (seq_length, batch_size, hidden_dim) got {input.shape}"
+        x = self.ln_1(input)
+        x, _ = self.self_attention(query=x, key=x, value=x, need_weights=False)
+        x = self.dropout(x)
+        x = x + input
+
+        y = self.ln_2(x)
+        y = self.mlp(y)
+        return x + y
+
+
+class Encoder(nn.Module):
+    """Transformer Model Encoder for sequence to sequence translation."""
+
+    def __init__(
+        self,
+        seq_length: int,
+        num_layers: int,
+        num_heads: int,
+        hidden_dim: int,
+        mlp_dim: int,
+        dropout_rate: float,
+        attention_dropout_rate: float,
+    ):
+        super().__init__()
+        # Note that batch_size is on the second dim because
+        # we have batch_first=False in nn.MultiAttention() by default
+        self.pos_embedding = nn.Parameter(torch.empty(seq_length, 1, hidden_dim).normal_(std=0.02))  # from BERT
+        self.dropout = nn.Dropout(dropout_rate)
+        layers: OrderedDict[str, nn.Module] = OrderedDict()
+        for i in range(num_layers):
+            layers[f"encoder_layer_{i}"] = EncoderBlock(
+                num_heads,
+                hidden_dim,
+                mlp_dim,
+                dropout_rate,
+                attention_dropout_rate,
+            )
+        self.layers = nn.Sequential(layers)
+        self.ln = LayerNorm(hidden_dim)
+
+    def forward(self, input: Tensor):
+        # assert input.dim() == 3, f"Expected (seq_length, batch_size, hidden_dim) got {input.shape}"
+        input = input + self.pos_embedding
+        return self.ln(self.layers(self.dropout(input)))
+
+
+class VisionTransformer(nn.Module):
+    """Vision Transformer as per https://arxiv.org/abs/2010.11929."""
+
+    def __init__(
+        self,
+        image_size: int,
+        patch_size: int,
+        num_layers: int,
+        num_heads: int,
+        hidden_dim: int,
+        mlp_dim: int,
+        dropout_rate: float = 0.0,
+        attention_dropout_rate: float = 0.0,
+        classifier: str = "token",
+        num_classes: int = 1000,
+    ):
+        super().__init__()
+        # assert image_size % patch_size == 0, "Input shape indivisible by patch size!"
+        # assert classifier in ["token", "gap"], "Unexpected classifier mode!"
+        self.image_size = image_size
+        self.patch_size = patch_size
+        self.hidden_dim = hidden_dim
+        self.mlp_dim = mlp_dim
+        self.attention_dropout_rate = attention_dropout_rate
+        self.dropout_rate = dropout_rate
+        self.classifier = classifier
+        self.num_classes = num_classes
+
+        input_channels = 3
+
+        # The conv_proj is a more efficient version of reshaping, permuting
+        # and projecting the input
+        self.conv_proj = nn.Conv2d(input_channels, hidden_dim, kernel_size=patch_size, stride=patch_size)
+
+        seq_length = (image_size // patch_size) ** 2
+        if self.classifier == "token":
+            # Add a class token
+            self.class_token = nn.Parameter(torch.zeros(1, 1, hidden_dim))
+            seq_length += 1
+
+        self.encoder = Encoder(
+            seq_length,
+            num_layers,
+            num_heads,
+            hidden_dim,
+            mlp_dim,
+            dropout_rate,
+            attention_dropout_rate,
+        )
+        self.seq_length = seq_length
+
+        self.head = nn.Linear(hidden_dim, num_classes)
+        self.init_weights()
+
+    def init_weights(self):
+        fan_in = self.conv_proj.in_channels * self.conv_proj.kernel_size[0] * self.conv_proj.kernel_size[1]
+        nn.init.trunc_normal_(self.conv_proj.weight, std=math.sqrt(1 / fan_in))
+        nn.init.zeros_(self.conv_proj.bias)
+        nn.init.zeros_(self.head.weight)
+
+    def forward(self, x: torch.Tensor):
+        n, c, h, w = x.shape
+        p = self.patch_size
+        # assert h == w == self.image_size
+        n_h = h // p
+        n_w = w // p
+
+        # (n, c, h, w) -> (n, hidden_dim, n_h, n_w)
+        x = self.conv_proj(x)
+        # (n, hidden_dim, n_h, n_w) -> (n, hidden_dim, (n_h * n_w))
+        x = x.reshape(n, self.hidden_dim, n_h * n_w)
+
+        # (n, hidden_dim, (n_h * n_w)) -> ((n_h * n_w), n, hidden_dim)
+        # The self attention layer expects inputs in the format (S, N, E)
+        # where S is the source sequence length, N is the batch size, E is the
+        # embedding dimension
+        x = x.permute(2, 0, 1)
+
+        if self.classifier == "token":
+            # Expand the class token to the full batch.
+            batch_class_token = self.class_token.expand(-1, n, -1)
+            x = torch.cat([batch_class_token, x], dim=0)
+
+        x = self.encoder(x)
+
+        if self.classifier == "token":
+            # Classifier as used by standard language architectures
+            x = x[0, :, :]
+        elif self.classifier == "gap":
+            # Classifier as used by standard vision architectures
+            x = x.mean(dim=0)
+        else:
+            raise ValueError(f"Invalid classifier={self.classifier}")
+
+        x = self.head(x)
+
+        return x
+
+
+def _vision_transformer(version: str, pretrained: bool, progress: bool, **kwargs: Any) -> VisionTransformer:
+    if kwargs.get("image_size", None) is None:
+        model = VisionTransformer(image_size=224, **kwargs)
+    else:
+        model = VisionTransformer(**kwargs)
+    # TODO: Adding pre-trained models
+    return model
+
+
+def vit_b_16(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> VisionTransformer:
+    """
+    Constructs a ViT_b_16 architecture from
+    `"An Image is Worth 16x16 Words: Transformers for Image Recognition at Scale" <https://arxiv.org/abs/2010.11929>`_.
+
+    Args:
+        pretrained (bool): If True, returns a model pre-trained on ImageNet
+        progress (bool): If True, displays a progress bar of the download to stderr
+    """
+    return _vision_transformer(
+        version="b_16",
+        pretrained=pretrained,
+        progress=progress,
+        patch_size=16,
+        num_layers=12,
+        num_heads=12,
+        hidden_dim=768,
+        mlp_dim=3072,
+        **kwargs,
+    )
+
+
+def vit_b_32(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> VisionTransformer:
+    """
+    Constructs a ViT_b_32 architecture from
+    `"An Image is Worth 16x16 Words: Transformers for Image Recognition at Scale" <https://arxiv.org/abs/2010.11929>`_.
+
+    Args:
+        pretrained (bool): If True, returns a model pre-trained on ImageNet
+        progress (bool): If True, displays a progress bar of the download to stderr
+    """
+    return _vision_transformer(
+        version="b_32",
+        pretrained=pretrained,
+        progress=progress,
+        patch_size=32,
+        num_layers=12,
+        num_heads=12,
+        hidden_dim=768,
+        mlp_dim=3072,
+        **kwargs,
+    )
+
+
+def vit_l_16(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> VisionTransformer:
+    """
+    Constructs a ViT_l_16 architecture from
+    `"An Image is Worth 16x16 Words: Transformers for Image Recognition at Scale" <https://arxiv.org/abs/2010.11929>`_.
+
+    Args:
+        pretrained (bool): If True, returns a model pre-trained on ImageNet
+        progress (bool): If True, displays a progress bar of the download to stderr
+    """
+    return _vision_transformer(
+        version="l_16",
+        pretrained=pretrained,
+        progress=progress,
+        patch_size=16,
+        num_layers=24,
+        num_heads=16,
+        hidden_dim=1024,
+        mlp_dim=4096,
+        **kwargs,
+    )
+
+
+def vit_l_32(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> VisionTransformer:
+    """
+    Constructs a ViT_l_32 architecture from
+    `"An Image is Worth 16x16 Words: Transformers for Image Recognition at Scale" <https://arxiv.org/abs/2010.11929>`_.
+
+    Args:
+        pretrained (bool): If True, returns a model pre-trained on ImageNet
+        progress (bool): If True, displays a progress bar of the download to stderr
+    """
+    return _vision_transformer(
+        version="l_32",
+        pretrained=pretrained,
+        progress=progress,
+        patch_size=32,
+        num_layers=24,
+        num_heads=16,
+        hidden_dim=1024,
+        mlp_dim=4096,
+        **kwargs,
+    )