feat: Add FP4 (E2M1) KV Cache Support for MHA (#12612)

Signed-off-by: Ho-Ren (Jack) Chuang <horenchuang@bytedance.com>
Co-authored-by: Yichen Wang <yichen.wang@bytedance.com>

python/sglang/srt/model_executor/model_runner.py

@@ -1325,6 +1325,24 @@ class ModelRunner:
                 * 2
                 * torch._utils._element_size(self.kv_cache_dtype)
             )
+
+            if is_float4_e2m1fn_x2(self.kv_cache_dtype):
+                # kv_scale_buffer
+                scale_block_size = 16
+
+                n = self.model_config.get_num_kv_heads(get_attention_tp_size())
+                k = self.model_config.head_dim
+                cell_size = (cell_size // 2) + (
+                    (
+                        n
+                        * k
+                        * num_layers
+                        * 2
+                        * torch._utils._element_size(self.kv_cache_dtype)
+                    )
+                    // scale_block_size
+                )
+
         rest_memory = available_gpu_memory - total_gpu_memory * (
             1 - self.mem_fraction_static
         )