Initial commit for faster rcnn resnet v1 fpn feature extractor.

1. Setup code structure for fpn feature extractor

research/object_detection/models/faster_rcnn_resnet_v1_fpn_keras_feature_extractor.py

+"""Faster RCNN Keras-based Resnet v1 FPN Feature Extractor."""
+
+from object_detection.meta_architectures import faster_rcnn_meta_arch
+from object_detection.models import feature_map_generators
+from object_detection.models.keras_models import resnet_v1
+from object_detection.models.keras_models import model_utils
+from object_detection.utils import ops
+
+
+class FasterRCNNFPNKerasFeatureExtractor(
+    faster_rcnn_meta_arch.FasterRCNNKerasFeatureExtractor):
+  """Faster RCNN Feature Extractor using Keras-based Resnet v1  FPN features."""
+
+  def __init__(self, ...):
+    # TODO: constructor
+    pass
+
+  def build(self, ...):
+    # TODO: Build the structure, should be very similar as ssd_*_fpn_keras_feature_extractor.py
+    # ResNet-101 (object_detection.models.keras_models)
+    # object_detection.models.feature_map_generators
+    pass
+  
+  def preprocess(self, ...):
+    # TODO: should be the same as others
+    pass
+  
+  def _extract_proposal_features(self, ...):
+    # TODO: Extracts first stage RPN features
+    # Fpn_feature_levels 
+    pass
+  
+  def _extract_box_classifier_features(self, ...):
+    # TODO: Extracts second stage box classifier features.
+    pass
+  
+  def restore_from_classification_checkpoint_fn(self, ...):
+    # follow the none fpn version
+    pass