support IO,IOB,IOE,IOBES for sequence labeling & fix en mrc

Use ==/!= to compare str, bytes, and int literals

update paper arxiv link

Identity is not the same thing as equality in Python.

$ python
```
>>> self_dtype = "float"
>>> self_dtype += "16"
>>> self_dtype == "float16"
True
>>> self_dtype is "float16"
False
```

update .run_ce.sh

added space and fixed small error

update

Remove bert & elmo

added space and fixed small error

some update

update readme

update README

amend several chinese descriptions

Release ERNIE 2.0

ERNIE 2.0 is a continual pre-training framework for language understanding in which pre-training tasks can be incrementally built and learned through multi-task learning

add utils/init.py

test=develop
fix #215

fix #212

modify_readme,test=develop

test=develop

enable gc for predict_classify

add classify inference using infer_program

fix #207

add ce  for ELMo