[Fix] Gemma hidden_activation compatibility (#2614)

This PR fixes the Gemma config compatibility issue.

Author: MasterJH5574

python/mlc_llm/model/gemma/gemma_model.py

@@ -22,7 +22,6 @@ class GemmaConfig(ConfigBase):  # pylint: disable=too-many-instance-attributes
     """Configuration of the Gemma model."""
 
     hidden_size: int
-    hidden_activation: Optional[str]
     intermediate_size: int
     attention_bias: bool
     num_attention_heads: int
@@ -31,6 +30,7 @@ class GemmaConfig(ConfigBase):  # pylint: disable=too-many-instance-attributes
     num_hidden_layers: int
     rms_norm_eps: float
     vocab_size: int
+    hidden_activation: Optional[str] = None
     position_embedding_base: int = 0
     context_window_size: int = 0
     prefill_chunk_size: int = 0