Add some header comments to tensor runtime files

...ts/hpvm-tensor-rt/tensor_runtime/src/approx_techniques.cu

...s/hpvm-tensor-rt/tensor_runtime/src/half_precision_api.cu

.../hpvm-tensor-rt/tensor_runtime/src/hpvm-rt-controller.cpp

hpvm/projects/hpvm-tensor-rt/tensor_runtime/src/profiling.cc

...rojects/hpvm-tensor-rt/tensor_runtime/src/tensor_utils.cu