Fixup wrong key lookup. (#2048)

This is a follow up to #1948.

Author: RomeoV

lib/cudnn/src/convolution.jl

@@ -279,7 +279,7 @@ function cudnnConvolutionBwdFilterAlgoPerf(xDesc, x, dyDesc, dy, convDesc, dwDes
 
     key = (xDesc_native, dyDesc_native, convDesc_native)
     val = lock(cudnnConvolutionBwdFilterAlgoPerfCacheLock) do
-        get(cudnnConvolutionBwdFilterAlgoPerfCache, (xDesc, dyDesc, convDesc), nothing)
+        get(cudnnConvolutionBwdFilterAlgoPerfCache, key, nothing)
     end
     if val === nothing
         requestedAlgoCount = Int(CUDNN_CONVOLUTION_BWD_FILTER_ALGO_COUNT)