Nested Labels Integration

[kozlov721]

Integration of changes in luxonis-ml introduced in #211.

Config Changes

• Field task in node configs renamed to task_name for naming consistency with luxonis-ml

Example
Let's have a dataset with 3 tasks; motorbike and car for detection, and vehicles for semantic segmentation. The labels would have the following keys:

{
    "motorbike/boundingbox",
    "motorbike/keypoints",
    "car/boundingbox",
    "car/keypoints",
    "vehicles/segmentation",
}

The following configuration for luxonis-train would make use of the above tasks:

model:
  name: parking_lot_model
  nodes:

    - name: EfficientRep

    - name: RepPANNeck

    - name: EfficientKeypointBBoxHead
      task_name: motorbike

    - name: EfficientKeypointBBoxHead
      task_name: car

    - name: SegmentationHead
      task_name: vehicles

[klemen1999]

Generally LGTM, left some comments

[sokovninn]

LGTM

[JSabadin]

LGTM!