Add flake8 to CI about gluon-nlp HOT 2 CLOSED

I usually do it in two passes like this:

    # stop the build if there are Python syntax errors or undefined names
    - flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics
    # exit-zero treats all errors as warnings.  The GitHub editor is 127 chars wide
    - flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics

The first pass halts the build on error but the second just treats "style violations" as warnings.

E901,E999,F821,F822,F823 are the "showstopper" flake8 issues that can halt the runtime with a SyntaxError, NameError, etc. Most other flake8 issues are merely "style violations" -- useful for readability but they do not effect runtime safety. This PR should therefore recommend a flake8 run of these tests on the entire codebase.

• F821: undefined name name
• F822: undefined name name in __all__
• F823: local variable name referenced before assignment
• E901: SyntaxError or IndentationError
• E999: SyntaxError -- failed to compile a file into an Abstract Syntax Tree

from gluon-nlp.

Thanks. I believe we should enforce E901,E999,F821,F822,F823 on the CI. I have enabled the check for them following your suggestion in #213 (subject to review).
I would be happy to also enforce style related issues (once they are all fixed or selectively disabled), but let's see what others think.

from gluon-nlp.