Gemma2: fix FA2 generation (huggingface#32553)

fix FA2

src/transformers/models/gemma2/modeling_gemma2.py

@@ -1093,7 +1093,11 @@ def prepare_inputs_for_generation(
             # The clone here is for the same reason as for `position_ids`.
             model_inputs = {"input_ids": input_ids.clone(memory_format=torch.contiguous_format), "inputs_embeds": None}
 
-        if isinstance(past_key_values, HybridCache) and attention_mask.ndim == 2:
+        if (
+            isinstance(past_key_values, HybridCache)
+            and attention_mask.ndim == 2
+            and not self.config._attn_implementation == "flash_attention_2"
+        ):
             if model_inputs["inputs_embeds"] is not None:
                 batch_size, sequence_length, _ = model_inputs["inputs_embeds"].shape
                 device = model_inputs["inputs_embeds"].device