Created by: zhiqiu

Paddle uses contextlib.contextmanager internally to decorated several context managers. For example, a simple looks like:

var = 0
@contextlib.contextmanager
def guard(tracer):
    var = 1
    yield
    var = 0

with guard():
    # do something 
    # raise exception

Normally, when entering the with context, the code block before yield in function guard() is executed and var is set to 1; and when exiting the context, the code block after yield in function guard() is executed and var is set to 0.

However, when exception raised inside with context, the code block after yield in function guard() is cannot be executed and var cannot set back to 0. This may cause some errors and problems.

One typical problem is in unittest, if one unittest that using context manager failed, the others will continues to be executed, with the wrong context.

This PR add try...finally... to exit the context manager safely, and solve the problems discussed above.