[mkldnn-v1.0] Update enabling flag for MKL dropout (#16433)

* use MSHADOW_USE_MKL to determine whther to use mkl optimized dropout

* rebase code

Author: wuxun-zhang

src/operator/nn/dropout-inl.h

@@ -39,7 +39,7 @@
 #include "../random/sampler.h"
 #include "../tensor/elemwise_binary_broadcast_op.h"
 
-#if defined(USE_MKL) && defined(_OPENMP) && !defined(__CUDACC__)
+#if (MSHADOW_USE_MKL == 1) && defined(_OPENMP) && !defined(__CUDACC__)
 #define MXNET_USE_MKL_DROPOUT 1
 #endif