onnxruntime

Fix unused variable for CUDA EP builds with USE_FLASH_ATTENTION off (#14404)

### Description
Fixes unused `use_memory_efficient_attention` variable in
contrib_ops/cuda/bert/attention_impl.cu.



### Motivation and Context
ORT with CUDA version < 11.6 fails to build for release configurations
due to an unused variable.

```shell
c:\...\onnxruntime\onnxruntime\contrib_ops\cuda\bert\attention_impl.cu(420): error : variable "use_memory_efficient_attention" was declared but never referenced [C:\...\onnxruntime\build\Windows\RelWithDebInfo\onnx
runtime_providers_cuda.vcxproj]
            detected during instantiation of "onnxruntime::common::Status onnxruntime::contrib::cuda::QkvToContext(const cudaDeviceProp &, cublasHandle_t &, cudaStream_t, onnxruntime::contrib::AttentionParameters &, onnxruntime::contrib::cuda::AttentionData<T> &) [wit
  h T=float]"
  (923): here
```

This happens for CUDA < 11.6. Our cmake script turns off
onnxruntime_USE_FLASH_ATTENTION for CUDA < 11.6, which leaves the
aforementioned variable unused outside of asserts (which are removed in
release builds).

The USE_FLASH_ATTENTION option was added by
https://github.com/microsoft/onnxruntime/pull/14343