Make dlib/__init__.py know the paths to cuda that cmake used. Use those

paths instead of relying on CUDA_PATH to be set.

tools/python/CMakeLists.txt

@@ -71,3 +71,4 @@ endif()
 pybind11_add_module(_dlib_pybind11 ${python_srcs})
 target_link_libraries(_dlib_pybind11 PRIVATE dlib::dlib)
 
+configure_file(${PROJECT_SOURCE_DIR}/dlib/__init__.py.in ${CMAKE_LIBRARY_OUTPUT_DIRECTORY}/dlib/__init__.py)

tools/python/dlib/__init__.py

@@ -1,13 +0,0 @@
-# Copyright (C) 2020  Davis E. King (davis@dlib.net)
-# License: Boost Software License   See LICENSE.txt for the full license.
-
-try:
-    import os
-    # On windows you must call os.add_dll_directory() to allow linking to external DLLs.  See
-    # https://docs.python.org/3.8/whatsnew/3.8.html#bpo-36085-whatsnew.
-    os.add_dll_directory(os.path.join(os.environ['CUDA_PATH'], 'bin'))
-except (AttributeError,KeyError):
-    pass
-
-from _dlib_pybind11 import *
-from _dlib_pybind11 import __version__, __time_compiled__

tools/python/dlib/__init__.py.in

@@ -0,0 +1,20 @@
+# Copyright (C) 2020  Davis E. King (davis@dlib.net)
+# License: Boost Software License   See LICENSE.txt for the full license.
+
+def add_lib_to_dll_path(path):
+    """ On windows you must call os.add_dll_directory() to allow linking to external DLLs.  See
+    https://docs.python.org/3.8/whatsnew/3.8.html#bpo-36085-whatsnew.  This function adds the folder
+    containing path to the dll search path. 
+    """
+    try:
+        import os
+        os.add_dll_directory(os.path.dirname(path))
+    except (AttributeError,KeyError):
+        pass
+
+if '@DLIB_USE_CUDA@' == 'ON':
+    add_lib_to_dll_path('@cudnn@')
+    add_lib_to_dll_path('@CUDA_CUDART_LIBRARY@')
+
+from _dlib_pybind11 import *
+from _dlib_pybind11 import __version__, __time_compiled__