Give tf.contrib.summary a default suffix of .v2. This is so that it doesn't...

Give tf.contrib.summary a default suffix of .v2. This is so that it doesn't conflict with v1 summaries that are being created at the same time.

By default, MonitoredTrainingSession and Estimator will create v1 summaries, so using both currently creates a race condition, and the last one to create the event file writer wins.

PiperOrigin-RevId: 183919988