Created by: qingqing01

Fix #3883 (closed)

• Correctly use host_vector

  • If use the default allocator in host_vector, the LoD information can not be accessed in CUDA kernel. So use the thrust::system::cuda::experimental::pinned_allocator<T> allocator.
  • Add unit testing lod_tensor_test.cu to test LoDTensor for GPU.

• Expose LoDTensor to Python.

  • Expose LoDTensor in pybind.
  • Add unit testing.