LoDTensor Python API Change To-Do List

[kexinzhao]

The previous work has been summarized in #11106.

Based on the @panyx0718 's comment, there are some remaining work to do:

1. In Fluid's Python code (especially the op test code), there are variable's of name lod like the following:
   

   Paddle/python/paddle/fluid/data_feeder.py

   Line 54 in c660b47

   self.lod = []

Since we are replacing the notion of lod with recursive_sequence_lengths, we want to change these variable names from lod to recursive_sequence_lengths or similar shorter name.

2. Change the lod_tensor.md to reflect the change in pybind API.

3. Change the mention of name lod to recursive_sequence_lengths in the document comments in python files including nn.py and control_flow.py.

4. There are another concept called lod_level which is essentially the depth of lod, so we need to repeat item 1-3 to replace the notion lod_level with something like sequence_lengths_depth.

5. Remove the lod, set_lod deprecated API in pybind in the future.

[shanyi15]

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