switch to rotary embeddings, as they did in the paper

Author: lucidrains

MEGABYTE_pytorch/attend.py

@@ -89,34 +89,19 @@ def flash_attn(self, q, k, v, mask = None, attn_bias = None):
 
         config = self.cuda_config if is_cuda else self.cpu_config
 
-        causal = self.causal
-
-        # handle attention bias
-
-        if exists(attn_bias):
-            mask_value = -torch.finfo(q.dtype).max // 2
-            causal_mask = self.get_mask(q_len, k_len, device)
-            attn_bias = attn_bias.masked_fill(causal_mask, mask_value)
-
-            if exists(mask):
-                attn_bias = attn_bias.masked_fill(~mask, mask_value)
-
-            mask = attn_bias
-            causal = False
-
         # pytorch 2.0 flash attn: q, k, v, mask, dropout, causal, softmax_scale
 
         with torch.backends.cuda.sdp_kernel(**config._asdict()):
             out = F.scaled_dot_product_attention(
                 q, k, v,
                 attn_mask = mask,
                 dropout_p = self.dropout if self.training else 0., 
-                is_causal = causal
+                is_causal = self.causal
             )
 
         return out
 
-    def forward(self, q, k, v, mask = None, attn_bias = None):
+    def forward(self, q, k, v, mask = None):
         """
         einstein notation
         b - batch
@@ -132,17 +117,12 @@ def forward(self, q, k, v, mask = None, attn_bias = None):
         kv_einsum_eq = 'b j d' if k.ndim == 3 else 'b h j d'
 
         if self.flash:
-            return self.flash_attn(q, k, v, mask = mask, attn_bias = attn_bias)
+            return self.flash_attn(q, k, v, mask = mask)
 
         # similarity
 
         sim = einsum(f"b h i d, {kv_einsum_eq} -> b h i j", q, k) * scale
 
-        # attention bias
-
-        if exists(attn_bias):
-            sim = sim + attn_bias
-
         # causal mask
 
         if self.causal:

MEGABYTE_pytorch/megabyte.py

@@ -66,40 +66,30 @@ def token_shift(t):
     t_shift = F.pad(t_shift, (0, 0, 1, -1))
     return torch.cat((t, t_shift), dim = -1)
 
-# positional bias
+# rotary positional embedding
 
-class Alibi(nn.Module):
-    def __init__(self, heads, **kwargs):
+class RotaryEmbedding(nn.Module):
+    def __init__(self, dim, theta = 10000):
         super().__init__()
-        self.heads = heads
-        slopes = torch.Tensor(self._get_slopes(heads))
-        slopes = rearrange(slopes, 'h -> h 1 1')
-        self.register_buffer('slopes', slopes, persistent = False)
-        self.register_buffer('bias', None, persistent = False)
-
-    @staticmethod
-    def _get_slopes(heads):
-        def get_slopes_power_of_2(n):
-            start = (2**(-2**-(math.log2(n)-3)))
-            ratio = start
-            return [start*ratio**i for i in range(n)]
+        inv_freq = 1.0 / (theta ** (torch.arange(0, dim, 2).float() / dim))
+        self.register_buffer("inv_freq", inv_freq)
 
-        if math.log2(heads).is_integer():
-            return get_slopes_power_of_2(heads)
+    @property
+    def device(self):
+        return next(self.buffers()).device
 
-        closest_power_of_2 = 2 ** math.floor(math.log2(heads))
-        return get_slopes_power_of_2(closest_power_of_2) + get_slopes_power_of_2(2 * closest_power_of_2)[0::2][:heads-closest_power_of_2]
+    def forward(self, seq_len):
+        t = torch.arange(seq_len, device = self.device).type_as(self.inv_freq)
+        freqs = torch.einsum('i , j -> i j', t, self.inv_freq)
+        freqs = torch.cat((freqs, freqs), dim = -1)
+        return freqs
 
-    def forward(self, i, j, device):
-        if exists(self.bias) and self.bias.shape[-1] >= j:
-            return self.bias[..., :j]
+def rotate_half(x):
+    x1, x2 = x.chunk(2, dim=-1)
+    return torch.cat((-x2, x1), dim=-1)
 
-        bias = torch.arange(j, device = device)
-        bias = rearrange(bias, 'j -> 1 1 j')
-        bias = bias * self.slopes
-
-        self.register_buffer('bias', bias, persistent = False)
-        return self.bias
+def apply_rotary_pos_emb(pos, t):
+    return t * pos.cos() + rotate_half(t) * pos.sin()
 
 # norm
 
@@ -152,14 +142,17 @@ def __init__(
         self.to_kv = nn.Linear(dim, dim_head * 2, bias = False)
         self.to_out = nn.Linear(inner_dim, dim, bias = False)
 
-    def forward(self, x, attn_bias = None):
+    def forward(self, x, rotary_emb = None):
         h, device = self.heads, x.device
 
         x = self.norm(x)
         q, k, v = (self.to_q(x), *self.to_kv(x).chunk(2, dim = -1))
         q = rearrange(q, 'b n (h d) -> b h n d', h = h)
 
-        out = self.attend(q, k, v, attn_bias = attn_bias)
+        if exists(rotary_emb):
+            q, k = map(lambda t: apply_rotary_pos_emb(rotary_emb, t), (q, k))
+
+        out = self.attend(q, k, v)
 
         out = rearrange(out, 'b h n d -> b n (h d)')
         return self.to_out(out)
@@ -175,11 +168,11 @@ def __init__(
         attn_dropout = 0.,
         ff_dropout = 0.,
         ff_mult = 4,
-        rel_pos_bias = True,
+        rel_pos = True,
         flash_attn = False
     ):
         super().__init__()
-        self.alibi = Alibi(heads = heads) if rel_pos_bias else None
+        self.rotary_emb = RotaryEmbedding(dim_head) if rel_pos else None
         self.layers = nn.ModuleList([])
 
         for _ in range(layers):
@@ -192,10 +185,10 @@ def __init__(
 
     def forward(self, x):
         n = x.shape[-2]
-        attn_bias = self.alibi(n, n, device = x.device) if exists(self.alibi) else None
+        rotary_emb = self.rotary_emb(n) if exists(self.rotary_emb) else None
 
         for attn, ff in self.layers:
-            x = attn(token_shift(x), attn_bias = attn_bias) + x
+            x = attn(token_shift(x), rotary_emb = rotary_emb) + x
             x = ff(token_shift(x)) + x
 
         return self.norm(x)
@@ -218,7 +211,7 @@ def __init__(
         ff_mult = 4,
         ff_dropout = 0.,
         pad_id = 0,
-        rel_pos_bias = False,
+        rel_pos = False,
         pos_emb = False,
         flash_attn = False
     ):
@@ -264,7 +257,7 @@ def __init__(
                 attn_dropout = attn_dropout,
                 ff_dropout = ff_dropout,
                 ff_mult = ff_mult,
-                rel_pos_bias = rel_pos_bias,
+                rel_pos = rel_pos,
                 flash_attn = flash_attn
             ))
 

README.md

@@ -89,15 +89,6 @@ $ python train.py
 }
 ```
 
-```bibtex
-@misc{press2021ALiBi,
-    title   = {Train Short, Test Long: Attention with Linear Biases Enable Input Length Extrapolation},
-    author  = {Ofir Press and Noah A. Smith and Mike Lewis},
-    year    = {2021},
-    url     = {https://ofir.io/train_short_test_long.pdf}
-}
-```
-
 ```bibtex
 @software{peng_bo_2021_5196578,
     author    = {PENG Bo},
@@ -120,3 +111,14 @@ $ python train.py
     volume  = {abs/2305.19466}
 }
 ```
+
+```bibtex
+@misc{su2021roformer,
+    title   = {RoFormer: Enhanced Transformer with Rotary Position Embedding},
+    author  = {Jianlin Su and Yu Lu and Shengfeng Pan and Bo Wen and Yunfeng Liu},
+    year    = {2021},
+    eprint  = {2104.09864},
+    archivePrefix = {arXiv},
+    primaryClass = {cs.CL}
+}
+```

setup.py

@@ -3,7 +3,7 @@
 setup(
   name = 'MEGABYTE-pytorch',
   packages = find_packages(),
-  version = '0.1.6',
+  version = '0.1.7',
   license='MIT',
   description = 'MEGABYTE - Pytorch',
   long_description_content_type = 'text/markdown',