diff --git a/README.md b/README.md
index 1eb3abb..edb97ee 100644
--- a/README.md
+++ b/README.md
@@ -24,6 +24,7 @@
- [Simple Masked Image Modeling](#simple-masked-image-modeling)
- [Masked Patch Prediction](#masked-patch-prediction)
- [Adaptive Token Sampling](#adaptive-token-sampling)
+- [Patch Merger](#patch-merger)
- [Vision Transformer for Small Datasets](#vision-transformer-for-small-datasets)
- [Dino](#dino)
- [Accessing Attention](#accessing-attention)
@@ -732,12 +733,58 @@ v = ViT(
img = torch.randn(4, 3, 256, 256)
-preds = v(img) # (1, 1000)
+preds = v(img) # (4, 1000)
# you can also get a list of the final sampled patch ids
# a value of -1 denotes padding
-preds, token_ids = v(img, return_sampled_token_ids = True) # (1, 1000), (1, <=8)
+preds, token_ids = v(img, return_sampled_token_ids = True) # (4, 1000), (4, <=8)
+```
+
+## Patch Merger
+
+
+
+
+This paper proposes a simple module (Patch Merger) for reducing the number of tokens at any layer of a vision transformer without sacrificing performance.
+
+```python
+import torch
+from vit_pytorch.vit_with_patch_merger import ViT
+
+v = ViT(
+ image_size = 256,
+ patch_size = 16,
+ num_classes = 1000,
+ dim = 1024,
+ depth = 12,
+ heads = 8,
+ patch_merge_layer = 6, # at which transformer layer to do patch merging
+ patch_merge_num_tokens = 8, # the output number of tokens from the patch merge
+ mlp_dim = 2048,
+ dropout = 0.1,
+ emb_dropout = 0.1
+)
+
+img = torch.randn(4, 3, 256, 256)
+
+preds = v(img) # (4, 1000)
+```
+
+One can also use the `PatchMerger` module by itself
+
+```python
+import torch
+from vit_pytorch.vit_with_patch_merger import PatchMerger
+
+merger = PatchMerger(
+ dim = 1024,
+ num_tokens_out = 8 # output number of tokens
+)
+
+features = torch.randn(4, 256, 1024) # (batch, num tokens, dimension)
+
+out = merger(features) # (4, 8, 1024)
```
## Vision Transformer for Small Datasets
@@ -1294,6 +1341,17 @@ Coming from computer vision and new to transformers? Here are some resources tha
}
```
+```bibtex
+@misc{renggli2022learning,
+ title = {Learning to Merge Tokens in Vision Transformers},
+ author = {Cedric Renggli and André Susano Pinto and Neil Houlsby and Basil Mustafa and Joan Puigcerver and Carlos Riquelme},
+ year = {2022},
+ eprint = {2202.12015},
+ archivePrefix = {arXiv},
+ primaryClass = {cs.CV}
+}
+```
+
```bibtex
@misc{vaswani2017attention,
title = {Attention Is All You Need},
diff --git a/images/patch_merger.png b/images/patch_merger.png
new file mode 100644
index 0000000..b7a537c
Binary files /dev/null and b/images/patch_merger.png differ
diff --git a/setup.py b/setup.py
index 122e5b0..9f50a6d 100644
--- a/setup.py
+++ b/setup.py
@@ -3,7 +3,7 @@ from setuptools import setup, find_packages
setup(
name = 'vit-pytorch',
packages = find_packages(exclude=['examples']),
- version = '0.26.7',
+ version = '0.27.0',
license='MIT',
description = 'Vision Transformer (ViT) - Pytorch',
author = 'Phil Wang',
diff --git a/vit_pytorch/vit_with_patch_merger.py b/vit_pytorch/vit_with_patch_merger.py
new file mode 100644
index 0000000..3106bb3
--- /dev/null
+++ b/vit_pytorch/vit_with_patch_merger.py
@@ -0,0 +1,144 @@
+import torch
+from torch import nn
+
+from einops import rearrange, repeat
+from einops.layers.torch import Rearrange, Reduce
+
+# helpers
+
+def exists(val):
+ return val is not None
+
+def default(val ,d):
+ return val if exists(val) else d
+
+def pair(t):
+ return t if isinstance(t, tuple) else (t, t)
+
+# patch merger class
+
+class PatchMerger(nn.Module):
+ def __init__(self, dim, num_tokens_out):
+ super().__init__()
+ self.scale = dim ** -0.5
+ self.norm = nn.LayerNorm(dim)
+ self.queries = nn.Parameter(torch.randn(num_tokens_out, dim))
+
+ def forward(self, x):
+ x = self.norm(x)
+ sim = torch.matmul(self.queries, x.transpose(-1, -2)) * self.scale
+ attn = sim.softmax(dim = -1)
+ return torch.matmul(attn, x)
+
+# classes
+
+class PreNorm(nn.Module):
+ def __init__(self, dim, fn):
+ super().__init__()
+ self.norm = nn.LayerNorm(dim)
+ self.fn = fn
+ def forward(self, x, **kwargs):
+ return self.fn(self.norm(x), **kwargs)
+
+class FeedForward(nn.Module):
+ def __init__(self, dim, hidden_dim, dropout = 0.):
+ super().__init__()
+ self.net = nn.Sequential(
+ nn.Linear(dim, hidden_dim),
+ nn.GELU(),
+ nn.Dropout(dropout),
+ nn.Linear(hidden_dim, dim),
+ nn.Dropout(dropout)
+ )
+ def forward(self, x):
+ return self.net(x)
+
+class Attention(nn.Module):
+ def __init__(self, dim, heads = 8, dim_head = 64, dropout = 0.):
+ super().__init__()
+ inner_dim = dim_head * heads
+ project_out = not (heads == 1 and dim_head == dim)
+
+ self.heads = heads
+ self.scale = dim_head ** -0.5
+
+ self.attend = nn.Softmax(dim = -1)
+ self.to_qkv = nn.Linear(dim, inner_dim * 3, bias = False)
+
+ self.to_out = nn.Sequential(
+ nn.Linear(inner_dim, dim),
+ nn.Dropout(dropout)
+ ) if project_out else nn.Identity()
+
+ def forward(self, x):
+ qkv = self.to_qkv(x).chunk(3, dim = -1)
+ q, k, v = map(lambda t: rearrange(t, 'b n (h d) -> b h n d', h = self.heads), qkv)
+
+ dots = torch.matmul(q, k.transpose(-1, -2)) * self.scale
+
+ attn = self.attend(dots)
+
+ out = torch.matmul(attn, v)
+ out = rearrange(out, 'b h n d -> b n (h d)')
+ return self.to_out(out)
+
+class Transformer(nn.Module):
+ def __init__(self, dim, depth, heads, dim_head, mlp_dim, dropout = 0., patch_merge_layer = None, patch_merge_num_tokens = 8):
+ super().__init__()
+ self.layers = nn.ModuleList([])
+
+ self.patch_merge_layer_index = default(patch_merge_layer, depth // 2) - 1 # default to mid-way through transformer, as shown in paper
+ self.patch_merger = PatchMerger(dim = dim, num_tokens_out = patch_merge_num_tokens)
+
+ for _ in range(depth):
+ self.layers.append(nn.ModuleList([
+ PreNorm(dim, Attention(dim, heads = heads, dim_head = dim_head, dropout = dropout)),
+ PreNorm(dim, FeedForward(dim, mlp_dim, dropout = dropout))
+ ]))
+ def forward(self, x):
+ for index, (attn, ff) in enumerate(self.layers):
+ x = attn(x) + x
+ x = ff(x) + x
+
+ if index == self.patch_merge_layer_index:
+ x = self.patch_merger(x)
+
+ return x
+
+class ViT(nn.Module):
+ def __init__(self, *, image_size, patch_size, num_classes, dim, depth, heads, mlp_dim, patch_merge_layer = None, patch_merge_num_tokens = 8, channels = 3, dim_head = 64, dropout = 0., emb_dropout = 0.):
+ super().__init__()
+ image_height, image_width = pair(image_size)
+ patch_height, patch_width = pair(patch_size)
+
+ assert image_height % patch_height == 0 and image_width % patch_width == 0, 'Image dimensions must be divisible by the patch size.'
+
+ num_patches = (image_height // patch_height) * (image_width // patch_width)
+ patch_dim = channels * patch_height * patch_width
+
+ self.to_patch_embedding = nn.Sequential(
+ Rearrange('b c (h p1) (w p2) -> b (h w) (p1 p2 c)', p1 = patch_height, p2 = patch_width),
+ nn.Linear(patch_dim, dim),
+ )
+
+ self.pos_embedding = nn.Parameter(torch.randn(1, num_patches + 1, dim))
+ self.dropout = nn.Dropout(emb_dropout)
+
+ self.transformer = Transformer(dim, depth, heads, dim_head, mlp_dim, dropout, patch_merge_layer, patch_merge_num_tokens)
+
+ self.mlp_head = nn.Sequential(
+ Reduce('b n d -> b d', 'mean'),
+ nn.LayerNorm(dim),
+ nn.Linear(dim, num_classes)
+ )
+
+ def forward(self, img):
+ x = self.to_patch_embedding(img)
+ b, n, _ = x.shape
+
+ x += self.pos_embedding[:, :n]
+ x = self.dropout(x)
+
+ x = self.transformer(x)
+
+ return self.mlp_head(x)