Update tf2_image_retraining.ipynb
Update tf2_image_retraining.ipynb

Changed `tf.test.is_gpu_available()` to `tf.config.list_physical_devices('GPU')`
Changed `lr` to `learning_rate`
Removes a deprecation warning. Tested the [updated .ipynb](https://colab.research.google.com/gist/chunduriv/1c4655040f81900f7f361ae1d187ae81/tf2_image_retraining.ipynb) file.
tensorflow/docs-l10n