[GSoC] TF 2.0 Mask R-CNN

[Vishal-V]

• TF 2.0 compatible Mask R-CNN model adapted from the Matterport implementation.
• Performance improvements with lesser parameters and improved residual blocks.
• Parallelized training with GPUs and an assortment of utils for various use cases.
• Associated Migration guide for the Tensorflow 2.0 specific changes.
• Runner script to run the model and draw inferences form the pre-trained COCO weights.