I want to know where to download cuDNN5.1 for TX2(arm64)? I can not find it in the DOWNLOAD page
cuDNN for arm64 is provided through JetPack. JetPack will install it to the TX2 for you, or you can run the first JetPack steps where it downloads the packages under the jetpack_downloads/ folder on the host PC, where you can then take these packages yourself.
The version of JetPack with cuDNN 5.1 was JetPack 3.0: https://developer.nvidia.com/embedded/jetpack-3_0
When running the JetPack installer, select JetPack 3.0 from the drop-down in the upper-left corner.