[cherry-pick 2.0 beta][Dy2Stat-debugging] Fix bug: Returns True if the verbosity level set by the user is greater than or equal to the log level (#26858) (#27052)

Change-Id: Iea4eb55f733e4751cb58c3caef35cb7664d2efa6

* [Dy2Stat] Add debugging and logging mechanism for dygraph to static. 

* Remove TransformerError temporarily. 

* import mock in PY2, from unittest import mock in PY3. test=develop

* Expose interfaces set_code_level and set_verbosity in paddle.jit, fix doc of the two interface. 

* polish doc of set_verbosity and set_code_level.