[XPU] fix dense tensor usage in adam (#70856)

Author: houj04

paddle/phi/kernels/xpu/adam_kernel.cc

@@ -77,8 +77,8 @@ void AdamDenseKernel(
 
   float* beta1_pow_ptr = nullptr;
   const float* beta1_const_pow_ptr = nullptr;
+  DenseTensor xpu_beta1_pow;
   if (beta1_pow.place() == CPUPlace()) {
-    DenseTensor xpu_beta1_pow;
     phi::Copy(dev_ctx, beta1_pow, dev_ctx.GetPlace(), false, &xpu_beta1_pow);
     if (xpu_beta1_pow.dtype() == DataType::FLOAT16)
       funcs::GetDataPointer<Context, float>(
@@ -95,8 +95,8 @@ void AdamDenseKernel(
 
   float* beta2_pow_ptr = nullptr;
   const float* beta2_const_pow_ptr = nullptr;
+  DenseTensor xpu_beta2_pow;
   if (beta2_pow.place() == CPUPlace()) {
-    DenseTensor xpu_beta2_pow;
     phi::Copy(dev_ctx, beta2_pow, dev_ctx.GetPlace(), false, &xpu_beta2_pow);
     if (xpu_beta2_pow.dtype() == DataType::FLOAT16)
       funcs::GetDataPointer<Context, float>(