Add _major_api_version setting to tensorflow top level module in pip package.

This attribute is checked in tensorflow/tools/api/tests:module_test and
tensorfow/tools/api/tests:api_compatibility_test.

PiperOrigin-RevId: 288046622
Change-Id: I9b8cf736a0395b9e90960567b0685db9b5307583

tensorflow/virtual_root_template_v1.__init__.py

@@ -100,6 +100,8 @@ for _m in _top_level_modules:
 # We still need all the names that are toplevel on tensorflow_core
 from tensorflow_core import *
 
+_major_api_version = 1
+
 # In V1 API we need to print deprecation messages
 from tensorflow.python.util import deprecation_wrapper as _deprecation
 if not isinstance(_sys.modules[__name__], _deprecation.DeprecationWrapper):

tensorflow/virtual_root_template_v2.__init__.py

@@ -100,6 +100,8 @@ for _m in _top_level_modules:
 # We still need all the names that are toplevel on tensorflow_core
 from tensorflow_core import *
 
+_major_api_version = 2
+
 # These should not be visible in the main tf module.
 try:
   del core