assertion is always true so remove the parentheses

[cclauss]

flake8 testing of https://github.com/youtubevos/MaskTrackRCNN on Python 3.7.1

$ flake8 . --count --select=E9,F63,F72,F82 --show-source --statistics

./mmdet/models/detectors/base.py:142:9: F631 assertion is always true, perhaps remove parentheses?
        assert(len(imgs) == 1, "only support mini-batch size 1") 
        ^
./mmdet/datasets/ytvos.py:98:30: F821 undefined name 'ExtraAugmentation'
            self.extra_aug = ExtraAugmentation(**extra_aug)
                             ^
1     F631 assertion is always true, perhaps remove parentheses?
1     F821 undefined name 'ExtraAugmentation'
2

E901,E999,F821,F822,F823 are the "showstopper" flake8 issues that can halt the runtime with a SyntaxError, NameError, etc. These 5 are different from most other flake8 issues which are merely "style violations" -- useful for readability but they do not effect runtime safety.

• 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

[linjieyangsc]

LGTM. Thanks for improving the repo!