Add doc on using LoRA for Gemma

PiperOrigin-RevId: 707548939

gemma/layers.py

@@ -22,10 +22,11 @@
 class Einsum(nn.Module):
   """Einsum is a convenience module for parameterized tensor multiplication."""
   shape: tuple[int, ...]
+  weight_name: str = 'w'
 
   @nn.compact
   def __call__(self, eqn: str, x: jax.Array) -> jax.Array:
-    w = self.param('w', nn.initializers.normal(), self.shape)
+    w = self.param(self.weight_name, nn.initializers.normal(), self.shape)
     return jnp.einsum(eqn, x, w)
 
 

gemma/modules.py

@@ -248,32 +248,31 @@ def __call__(self, x):
     # Some versions use an alternate parameter ordering that
     # transposes hidden_dim and features.
     if self.transpose_gating_einsum:
-      w_gating = self.param(
-          'gating_einsum',
-          nn.initializers.normal(),
-          ((2, self.hidden_dim, self.features)),
+      eq = '...F,NHF->...NH'
+      gating = layers.Einsum(
+          shape=(2, self.hidden_dim, self.features),
+          weight_name='gating_einsum',
       )
-      w_gating = w_gating.transpose((0, 2, 1))
     else:
-      w_gating = self.param(
-          'gating_einsum',
-          nn.initializers.normal(),
-          ((2, self.features, self.hidden_dim)),
+      eq = '...F,NFH->...NH'
+      gating = layers.Einsum(
+          shape=(2, self.features, self.hidden_dim),
+          weight_name='gating_einsum',
       )
-    ff_gate = jnp.dot(x, w_gating[0])
-    gate_value = nn.gelu(ff_gate)
 
-    # Up projection
-    ff1 = jnp.dot(x, w_gating[1])
-    activations = gate_value * ff1
+    # Use the same scope for backwards compatibility with existing checkpoints
+    # created before using `layers.Einsum` here.
+    nn.share_scope(self, gating)
+    gate = gating(eq, x)
+    activations = nn.gelu(gate[..., 0, :]) * gate[..., 1, :]
 
     # Down projection
-    w_linear = self.param(
-        'linear',
-        nn.initializers.zeros_init(),
-        (self.hidden_dim, self.features),
+    linear = layers.Einsum(
+        shape=(self.hidden_dim, self.features),
+        weight_name='linear',
     )
-    outputs = jnp.dot(activations, w_linear)
+    nn.share_scope(self, linear)
+    outputs = linear('...H,HF->...F', activations)
 
     return outputs