Fix: don't apply post layernorm in SiglipVisionTransformer (#2459)

* Fix: don't apply post layernorm in SiglipVisionTransformer

This fixes a bug with LLaVA Next when using Siglip as the vision model. LLaVA Next expects the output of the vision model to be the encoder outputs before layernorm (see original transformers implementation here: https://github.com/huggingface/transformers/blob/main/src/transformers/models/llava_next/modeling_llava_next.py#L813).

This also makes Siglip consistent with the existing Clip implementation:

https://github.com/huggingface/text-generation-inference/blob/main/server/text_generation_server/models/custom_modeling/clip.py#L613



* fix: adjust pali gemma for post layer norm and small refactors

---------
Co-authored-by: Travis Addair <tgaddair@gmail.com>

server/text_generation_server/models/custom_modeling/flash_pali_gemma_modeling.py

@@ -34,6 +34,11 @@ class PaliGemmaForConditionalGeneration(nn.Module):
             config=config.vision_config,
             weights=weights,
         )
+        self.post_vision_tower_layernorm = nn.LayerNorm.load(
+            prefix="vision_tower.vision_model.post_layernorm",
+            weights=weights,
+            eps=config.vision_config.layer_norm_eps,
+        )
 
         self.multi_modal_projector = TensorParallelColumnLinear.load(
             config,
@@ -84,7 +89,10 @@ class PaliGemmaForConditionalGeneration(nn.Module):
         if pixel_values is not None:
             pixel_values = pixel_values.to(dtype=inputs_embeds.dtype)
             image_outputs = self.vision_tower(pixel_values)
-            image_features = self.multi_modal_projector(image_outputs.last_hidden_state)
+            last_hidden_state = self.post_vision_tower_layernorm(
+                image_outputs.last_hidden_state
+            )
+            image_features = self.multi_modal_projector(last_hidden_state)
 
             # mask where image or padding tokens
             mask = input_ids == self.config.image_token_index

server/text_generation_server/models/custom_modeling/siglip.py

@@ -364,7 +364,6 @@ class SiglipEncoder(nn.Module):
         inputs_embeds,
         attention_mask: Optional[torch.Tensor] = None,
     ):
-
         hidden_states = inputs_embeds
         for idx, encoder_layer in enumerate(self.layers):
             hidden_states, _ = encoder_layer(
@@ -386,20 +385,11 @@ class SiglipVisionTransformer(nn.Module):
         self.encoder = SiglipEncoder(
             prefix=f"{prefix}.encoder", config=config, weights=weights
         )
-        self.post_layernorm = nn.LayerNorm.load(
-            prefix=f"{prefix}.post_layernorm",
-            weights=weights,
-            eps=config.layer_norm_eps,
-        )
 
     def forward(
         self,
         pixel_values: Optional[torch.FloatTensor] = None,
     ):
-        r"""
-        Returns:
-
-        """
         if pixel_values is None:
             raise ValueError("You have to specify pixel_values")
 
@@ -412,10 +402,9 @@ class SiglipVisionTransformer(nn.Module):
             inputs_embeds=hidden_states,
         )
         last_hidden_state = encoder_outputs
-        post_last_hidden_state = self.post_layernorm(last_hidden_state)
 
         return BaseModelOutputWithPooling(
-            last_hidden_state=post_last_hidden_state,
+            last_hidden_state=last_hidden_state,
             # pooler_output=pooled_output,
             # hidden_states=encoder_outputs,
         )