Feature/replaceable detection helper

[DXist]

These changes aim to make detectron more extensible as an external dependency.

1. model type func can be imported from environment by absolute python path. This allows to define helpers for custom architecture outside of the project directory. Particularly I'm interested to use ResNet18.
2. DetectionModelHelperclass can be replaced by custom implementation. In my case I want to override TrainableParams method to train only parameters related to mask.

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[DXist]

Fixed infer_simple test.

Test is run in custom docker image with fresh pytorch-nightly and python 3.7.