Fix issue 2385 - Refactoring needed for unfreeze_model(model) function in "Image classification via fine-tuning with EfficientNet" tutorial, "Transfer learning from pre-trained weights" section, second step

Fix issue 2385 - Refactoring needed for unfreeze_model(model) function in "Image classification via fine-tuning with EfficientNet" tutorial, "Transfer learning from pre-trained weights" section, second step #816