"frame_name" attr must be altered when importing/exporting MetaGraphDefs.

The frame_name attr of Enter operations must be the name of the
associated WhileLoopContext, otherwise taking the gradient of the loop
will result in frame errors.

I'm not happy that the export logic is in meta_graph.py (all other
control flow de/serialization is in control_flow_ops.py). However, I
can't think of how else to do it, since only export_scoped_meta_graph
has access to the NodeDefs being exported.

PiperOrigin-RevId: 184599323