Segmentation Models - v1.0.0

Areas of improvement

• Support for keras and tf.keras
• Losses as classes, base loss operations (sum of losses, multiplied loss)
• NCHW and NHWC support
• Removed pure tf operations to work with other keras backends
• Reduced a number of custom objects for better models serialization and deserialization

New featrues

• New backbones: EfficentNetB[0-7]
• New loss function: Focal loss
• New metrics: Precision, Recall

API changes

• get_preprocessing moved from sm.backbones.get_preprocessing to sm.get_preprocessing