[py_function] Don't attach py_function to the global eager graph.

Eager mode can incorrectly have a global graph.  Disabling global graph
on eager mode breaks too many assumptions so first introduce a flag indicating it.

Also, avoid attaching py_function to eager mode global graph, which is a leak.

Though this CL doesn't fix the leak yet as there are two more references that leads
to the leak, `tape_cache` and `ag_dnc_wrapper__` .

#35084

PiperOrigin-RevId: 288415011
Change-Id: Ica53e29521320af22c10609857d0a0219a9596ce

tensorflow/python/framework/ops.py

@@ -2785,6 +2785,11 @@ def __init__(self):
     # tuples: (input_shape_tuple, reduction_indices_tuple), and the values
     # are pairs of tuples: (output_shape_kept_dims, tile_scaling).
     self._reduced_shape_cache = {}
+    # In eager mode, the top level graph can still be created. This is
+    # incorrect and undesriable but currently so many places are relying on
+    # this. This is a flag indicating that, and meant to be set manually after
+    # this graph construction.
+    self._is_eager_graph = False
 
     # TODO(skyewm): fold as much of the above as possible into the C
     # implementation
@@ -5356,6 +5361,8 @@ def _GetGlobalDefaultGraph(self):
       #   the global default graph and an explicit graph are combined in the
       #   same process.
       self._global_default_graph = Graph()
+      if context.executing_eagerly():
+        self._global_default_graph._is_eager_graph = True  # pylint: disable=protected-access
     return self._global_default_graph
 
   def reset(self):

tensorflow/python/ops/script_ops.py

@@ -316,9 +316,11 @@ def _internal_py_func(func,
   while True:
     current_graph = graph
     if isinstance(graph, function._FuncGraph):  # pylint: disable=protected-access
-      graph = graph._outer_graph  # pylint: disable=protected-access
+      if not graph._outer_graph._is_eager_graph:  # pylint: disable=protected-access
+        graph = graph._outer_graph  # pylint: disable=protected-access
     elif isinstance(graph, func_graph.FuncGraph):
-      graph = graph.outer_graph
+      if not graph.outer_graph._is_eager_graph:  # pylint: disable=protected-access
+        graph = graph.outer_graph
     if graph is current_graph:
       break