How to filter predictions of specified classes 

## 📓 New <Tutorial/Example>
**Is this a request for a tutorial or for an example?**  
Tutorial.

**What is the task?**
Filter predictions of object detection model according to specified classes. 

**Is this example for a specific model?**
FasterRCNN

**Is this example for a specific dataset?**
Dataset is COCO.

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**Don't remove**
Main issue for examples: #39
