Revert "Fix FA4 import cause moe_fused_gate output be illegal memory" (#10432)

sgl-kernel/python/sgl_kernel/flash_attn.py

@@ -9,6 +9,11 @@ try:
 except:
     raise ImportError("Can not import sgl_kernel. Please check your installation.")
 
+try:
+    from ._fa4_interface import flash_attn_varlen_func as flash_attn_varlen_func_v4
+except ImportError:
+    flash_attn_varlen_func_v4 = None
+
 
 @lru_cache(maxsize=1)
 def is_fa3_supported(device=None) -> bool:
@@ -244,8 +249,9 @@ def flash_attn_varlen_func(
     ver=3,
 ):
     if ver == 4:
-        from ._fa4_interface import flash_attn_varlen_func as flash_attn_varlen_func_v4
-
+        assert (
+            flash_attn_varlen_func_v4 is not None
+        ), "FA4 is not available, please check your installation."
         # Using `(-1, -1)` as no sliding window causes correctness issues for FA4.
         if window_size == (-1, -1):
             window_size = (None, None)