add softcap interface for gemma-2

vllm/attention/backends/rocm_flash_attn.py

@@ -350,10 +350,17 @@ class ROCmFlashAttentionImpl(AttentionImpl):
         if blocksparse_params is not None:
             raise ValueError(
                 "ROCmFlashAttention does not support blocksparse attention.")
+        '''
         if logits_soft_cap is not None:
             raise ValueError(
                 "ROCmFlashAttention does not support attention logits soft "
                 "capping.")
+        '''
+        if logits_soft_cap is None:
+            # In flash-attn, setting logits_soft_cap as 0 means no soft cap.
+            logits_soft_cap = 0
+        self.logits_soft_cap = logits_soft_cap
+
         self.num_heads = num_heads
         self.head_size = head_size
         self.scale = float(scale)
@@ -566,6 +573,7 @@ class ROCmFlashAttentionImpl(AttentionImpl):
                         causal=True,
                         window_size=self.sliding_window,
                         alibi_slopes=self.alibi_slopes,
+                        softcap=self.logits_soft_cap,
                     )
 
                 # common code for prefill