F/flax split head dim (#5181)

* split_head_dim flax attn

* Make split_head_dim non default

* make style and make quality

* add description for split_head_dim flag

* Update src/diffusers/models/attention_flax.py
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

---------
Co-authored-by: Juan Acevedo <jfacevedo@google.com>
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

src/diffusers/models/attention_flax.py

@@ -131,6 +131,8 @@ class FlaxAttention(nn.Module):
             Dropout rate
         use_memory_efficient_attention (`bool`, *optional*, defaults to `False`):
             enable memory efficient attention https://arxiv.org/abs/2112.05682
+        split_head_dim (`bool`, *optional*, defaults to `False`):
+            Whether to split the head dimension into a new axis for the self-attention computation. In most cases, enabling this flag should speed up the computation for Stable Diffusion 2.x and Stable Diffusion XL.
         dtype (:obj:`jnp.dtype`, *optional*, defaults to jnp.float32):
             Parameters `dtype`
 
@@ -140,6 +142,7 @@ class FlaxAttention(nn.Module):
     dim_head: int = 64
     dropout: float = 0.0
     use_memory_efficient_attention: bool = False
+    split_head_dim: bool = False
     dtype: jnp.dtype = jnp.float32
 
     def setup(self):
@@ -177,6 +180,12 @@ class FlaxAttention(nn.Module):
         key_proj = self.key(context)
         value_proj = self.value(context)
 
+        if self.split_head_dim:
+            b = hidden_states.shape[0]
+            query_states = jnp.reshape(query_proj, (b, -1, self.heads, self.dim_head))
+            key_states = jnp.reshape(key_proj, (b, -1, self.heads, self.dim_head))
+            value_states = jnp.reshape(value_proj, (b, -1, self.heads, self.dim_head))
+        else:
             query_states = self.reshape_heads_to_batch_dim(query_proj)
             key_states = self.reshape_heads_to_batch_dim(key_proj)
             value_states = self.reshape_heads_to_batch_dim(value_proj)
@@ -206,14 +215,23 @@ class FlaxAttention(nn.Module):
             hidden_states = hidden_states.transpose(1, 0, 2)
         else:
             # compute attentions
+            if self.split_head_dim:
+                attention_scores = jnp.einsum("b t n h, b f n h -> b n f t", key_states, query_states)
+            else:
                 attention_scores = jnp.einsum("b i d, b j d->b i j", query_states, key_states)
+
             attention_scores = attention_scores * self.scale
-            attention_probs = nn.softmax(attention_scores, axis=2)
+            attention_probs = nn.softmax(attention_scores, axis=-1 if self.split_head_dim else 2)
 
             # attend to values
+            if self.split_head_dim:
+                hidden_states = jnp.einsum("b n f t, b t n h -> b f n h", attention_probs, value_states)
+                b = hidden_states.shape[0]
+                hidden_states = jnp.reshape(hidden_states, (b, -1, self.heads * self.dim_head))
+            else:
                 hidden_states = jnp.einsum("b i j, b j d -> b i d", attention_probs, value_states)
-
                 hidden_states = self.reshape_batch_dim_to_heads(hidden_states)
+
         hidden_states = self.proj_attn(hidden_states)
         return self.dropout_layer(hidden_states, deterministic=deterministic)