offer l2 distance attention for starters and cite

README.md

@@ -2239,6 +2239,16 @@ ids_out, num_out, is_number_mask = model.generate(start_ids, start_nums, 17)
     booktitle = {Neural Information Processing Systems},
     year    = {2018},
     url     = {https://api.semanticscholar.org/CorpusID:44064935}
+```
+
+```bibtex
+@article{Kim2020TheLC,
+    title   = {The Lipschitz Constant of Self-Attention},
+    author  = {Hyunjik Kim and George Papamakarios and Andriy Mnih},
+    journal = {ArXiv},
+    year    = {2020},
+    volume  = {abs/2006.04710},
+    url     = {https://api.semanticscholar.org/CorpusID:219530837}
 }
 ```
 

setup.py

@@ -3,7 +3,7 @@
 setup(
   name = 'x-transformers',
   packages = find_packages(exclude=['examples']),
-  version = '1.37.4',
+  version = '1.37.6',
   license='MIT',
   description = 'X-Transformers - Pytorch',
   author = 'Phil Wang',

tests/test_x_transformers.py

@@ -301,3 +301,22 @@ def test_sigsoftmax():
     model.eval()
 
     eval_logits = model(x)
+
+@pytest.mark.parametrize('attn_one_kv_head', (True, False))
+def test_l2_distance(attn_one_kv_head):
+
+    model = TransformerWrapper(
+        num_tokens = 20000,
+        max_seq_len = 1024,
+        attn_layers = Decoder(
+            dim = 512,
+            depth = 12,
+            heads = 8,
+            attn_l2_distance = True,
+            attn_one_kv_head = attn_one_kv_head,
+        )
+    )
+
+    x = torch.randint(0, 256, (1, 1024))
+
+    model(x)

x_transformers/attend.py

@@ -13,7 +13,7 @@
 from packaging import version
 from dataclasses import dataclass
 
-from einops import rearrange, repeat
+from einops import rearrange, repeat, pack, unpack
 
 # constants
 
@@ -39,9 +39,16 @@ def default(val, d):
 def compact(arr):
     return [*filter(exists, arr)]
 
-def softclamp(t, value):
+@torch.jit.script
+def softclamp(t: Tensor, value: float):
     return (t / value).tanh() * value
 
+def pack_one(t, pattern):
+    return pack([t], pattern)
+
+def unpack_one(t, ps, pattern):
+    return unpack(t, ps, pattern)[0]
+
 def once(fn):
     called = False
     @wraps(fn)
@@ -55,6 +62,18 @@ def inner(x):
 
 print_once = once(print)
 
+# alternative distance functions
+
+def qk_l2_dist_squared(q, k):
+    if k.ndim == 3:
+        k = repeat(k, 'b j d -> b h j d', h = q.shape[1])
+
+    q, packed_shape = pack_one(q, '* i d')
+    k, _ = pack_one(k, '* j d')
+
+    l2_dist_squared = torch.cdist(q, k) ** 2
+    return unpack_one(l2_dist_squared, packed_shape, '* i j')
+
 # functions for creating causal mask
 # need a special one for onnx cpu (no support for .triu)
 
@@ -80,6 +99,7 @@ def __init__(
         sparse_topk = None,
         scale = None,
         qk_norm = False,
+        l2_distance = False,
         flash = False,
         softclamp_logits = False,
         logit_softclamp_value = 50.,
@@ -123,6 +143,11 @@ def __init__(
         assert not (flash and sigsoftmax), 'sigsoftmax not available for flash attention'
         self.sigsoftmax = sigsoftmax
 
+        # l2 distance attention
+
+        assert not (flash and l2_distance), 'l2 distance attention does not work with flash attention just yet'
+        self.l2_distance = l2_distance
+
         # add a key / value token composed of zeros
         # in case this helps controlling outliers, proposed by https://www.evanmiller.org/attention-is-off-by-one.html
 
@@ -325,7 +350,12 @@ def forward(
 
         kv_einsum_eq = 'b j d' if k.ndim == 3 else 'b h j d'
 
-        sim = einsum(f'b h i d, {kv_einsum_eq} -> b h i j', q, k) * scale
+        if not self.l2_distance:
+            sim = einsum(f'b h i d, {kv_einsum_eq} -> b h i j', q, k)
+        else:
+            sim = -qk_l2_dist_squared(q, k)
+
+        sim = sim * scale
 
         if exists(prev_attn):
             sim = sim + prev_attn

x_transformers/x_transformers.py

@@ -923,6 +923,7 @@ def __init__(
         qk_norm_groups = 1,
         qk_norm_scale = 10,
         qk_norm_dim_scale = False,
+        l2_distance = False,
         one_kv_head = False,
         kv_heads = None,
         shared_kv = False,
@@ -1037,6 +1038,7 @@ def __init__(
             sparse_topk = sparse_topk,
             qk_norm = qk_norm,
             scale = qk_norm_scale if qk_norm else self.scale,
+            l2_distance = l2_distance,
             add_zero_kv = add_zero_kv,
             flash = flash,
             softclamp_logits = softclamp_logits,