Run flake8 tests to find syntax errors and undefined names

Created by: cclauss

Every time someone submits a pull request, Travis Continuous Integration can automatically run tests on the code before it is reviewed. Travis is always free to open source projects like this one but the owner of the this repo would need to go to https://travis-ci.org/profile and flip the repository switch on to enable testing.

flake8 testing of https://github.com/eriklindernoren/PyTorch-GAN on Python 3.6.3

$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

./implementations/dragan/dragan.py:201:9: F821 undefined name 'd_'
        d_
        ^
./implementations/pix2pix/models.py:15:21: E999 SyntaxError: invalid syntax
        if normalize=:
                    ^
1     E999 SyntaxError: invalid syntax
1     F821 undefined name 'd_'
2

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flake8 testing of https://github.com/eriklindernoren/Keras-GAN on Python 3.6.3

$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

./gan/gan_rgb.py:204:50: E999 TabError: inconsistent use of tabs and spaces in indentation
	        d_loss_logs_f_a = np.array(d_loss_logs_f)
                                                 ^
1     E999 TabError: inconsistent use of tabs and spaces in indentation
1