MirroredStrategy: Throw an error if there is a merge call in the middle of a...

MirroredStrategy: Throw an error if there is a merge call in the middle of a `fn` passed to call_for_each_replica which changes the graph being used while calling `fn`. This is because we switch from a replica thread to the main thread during a merge_call and changing the graph can have undesirable behavior currently.

PiperOrigin-RevId: 236022706