Cache the environment variable check for batch invariance (#26510)

Signed-off-by: Bram Wasti <bwasti@meta.com>

csrc/core/batch_invariant.hpp

@@ -8,9 +8,12 @@ namespace vllm {
 // vllm_kernel_override_batch_invariant(); returns true
 // if env VLLM_KERNEL_OVERRIDE_BATCH_INVARIANT=1
 inline bool vllm_kernel_override_batch_invariant() {
-  std::string env_key = "VLLM_KERNEL_OVERRIDE_BATCH_INVARIANT";
-  const char* val = std::getenv(env_key.c_str());
-  return (val && std::atoi(val) != 0) ? 1 : 0;
+  static bool cached = []() {
+    std::string env_key = "VLLM_KERNEL_OVERRIDE_BATCH_INVARIANT";
+    const char* val = std::getenv(env_key.c_str());
+    return (val && std::atoi(val) != 0) ? 1 : 0;
+  }();
+  return cached;
 }
 
 }  // namespace vllm