Created by: kexinzhao

Some background can be found here.

Essentially, our level of detail info (LoD) is currently both implemented in C++ and exposed in Python using "offsets". We want to encapsulate the offset implementation detail in C++ and only expose length based interface in Python.

What this PR does (See the main change in pybind.cc here):

1. Previously, the set_lod and lod API in pybind.cc uses offset_based LoD. This PR modifies the LoDTensor API in pybind.cc so that the user on the python side can provide or obtain length-based LoD using set_lod and lod, respectively.

2. The init API is also modified so that user can create a LoDTensor object in Python and initialize it with a length-based LoD.

3. A has_valid_lod API is added in pybind.cc so that user can check if the LoD info in a tensor is valid and matches the its data dimension on the python side.

4. Utility functions to create LoDTensor in the python side defined here is largely simplified using the updated set_lod and lod in pybind. (See changes here)

5. Over 30 tests that uses set_lod and lod has been modified to reflect the updated LoDTensor API in pybind.cc.