updated cudnn path (#2843)

* updated cudnn path

* adding cudnn path

* Use same format as command below

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Co-authored-by: Davis E. King <davis685@gmail.com>

dlib/cmake_utils/test_for_cudnn/find_cudnn.txt

@@ -4,7 +4,7 @@ message(STATUS "Looking for cuDNN install...")
 # libraries and also a few other places as well.
 find_path(cudnn_include cudnn.h
     HINTS ${CUDA_INCLUDE_DIRS} ENV CUDNN_INCLUDE_DIR  ENV CUDNN_HOME
-    PATHS /usr/local ENV CPATH
+    PATHS /usr/local /usr/local/cuda ENV CPATH
     PATH_SUFFIXES include
     )
 get_filename_component(cudnn_hint_path "${CUDA_CUBLAS_LIBRARIES}" PATH)