Switch TF to OSS keras (1/N).

1. Make api_template always use third_party/py/keras (or keras PIP package in OSS).
2. API tests are updated to ignore tensorflow/python/keras end points.
3. All Keras-related code will be removed in a follow up change. Keras-specific APIs are tracked in google3/third_party/py/keras/api/golden/
4. There are a few APIs (optimizer/initializer) that are exposed as both tf and keras APIs. Their pbtxt are updated to reflect the latest class path change.

Once this change is submitted, the current code in tensorflow/python/keras will become a stale copy, and will be removed soon. No more changes should be made to tensorflow/python/keras.

PiperOrigin-RevId: 379353790

research/object_detection/models/keras_models/resnet_v1.py

@@ -19,9 +19,10 @@ from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function
 
+from keras.applications import resnet
+
 import tensorflow.compat.v1 as tf
 
-from tensorflow.python.keras.applications import resnet
 from object_detection.core import freezable_batch_norm
 from object_detection.models.keras_models import model_utils