Add description of HPVM passes which deal with tensor operations

hpvm/lib/Transforms/DFG2LLVM_CUDNN/DFG2LLVM_CUDNN.cpp

...ib/Transforms/DFG2LLVM_WrapperAPI/DFG2LLVM_WrapperAPI.cpp

...ib/Transforms/FuseHPVMTensorNodes/FuseHPVMTensorNodes.cpp