fix function signature

csrc/amp_C_frontend.cpp

@@ -42,7 +42,7 @@ void multi_tensor_lamb_stage1_cuda(
     const float beta1,
     const float beta2,
     const float epsilon,
-    const float global_grad_norm,
+    at::Tensor global_grad_norm,
     const float max_global_grad_norm);
 
 void multi_tensor_lamb_stage2_cuda(
@@ -108,7 +108,7 @@ void multi_tensor_lamb_cuda(
   const float weight_decay,
   const int grad_averaging,
   const int mode,
-  const float global_grad_norm,
+  at::Tensor global_grad_norm,
   const float max_grad_norm,
   at::optional<bool> use_nvlamb_python);