Refactor object detection box predictors and fix some issues with model_main. (#4965)

* Merged commit includes the following changes:
206852642  by Zhichao Lu:

    Build the balanced_positive_negative_sampler in the model builder for FasterRCNN. Also adds an option to use the static implementation of the sampler.

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206803260  by Zhichao Lu:

    Fixes a misplaced argument in resnet fpn feature extractor.

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206682736  by Zhichao Lu:

    This CL modifies the SSD meta architecture to support both Slim-based and Keras-based box predictors, and begins preparation for Keras box predictor support in the other meta architectures.

    Concretely, this CL adds a new `KerasBoxPredictor` base class and makes the meta architectures appropriately call whichever box predictors they are using.

    We can switch the non-ssd meta architectures to fully support Keras box predictors once the Keras Convolutional Box Predictor CL is submitted.

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206669634  by Zhichao Lu:

    Adds an alternate m...

research/object_detection/protos/box_predictor.proto

@@ -93,6 +93,8 @@ message WeightSharedConvolutionalBoxPredictor {
   optional bool share_prediction_tower = 13 [default = false];
 }
 
+// TODO(alirezafathi): Refactor the proto file to be able to configure mask rcnn
+// head easily.
 message MaskRCNNBoxPredictor {
   // Hyperparameters for fully connected ops used in the box predictor.
   optional Hyperparams fc_hyperparams = 1;