issue/1033 skip build flash attn target when disabled

xmake.lua

@@ -461,7 +461,7 @@ target("infinicore_cpp_api")
     add_linkdirs(INFINI_ROOT.."/lib")
     add_links("infiniop", "infinirt", "infiniccl")
 
-    if get_config("flash-attn") ~= "" then
+    if get_config("flash-attn") ~= "" and get_config("flash-attn") ~= nil then
         add_installfiles("(builddir)/$(plat)/$(arch)/$(mode)/flash-attn*.so", {prefixdir = "lib"})
         if has_config("nv-gpu") then
             add_deps("flash-attn-nvidia")

xmake/nvidia.lua

@@ -181,6 +181,7 @@ target("flash-attn-nvidia")
         before_build(function (target)
             print("Flash Attention not available, skipping flash-attn-nvidia build")
         end)
+        on_build(function (target) end)
     end
 
     on_install(function (target) end)