This is the recommended way to check if a checkpoint exists, since it takes
into account the naming difference between V1 and V2 formats.
Args
checkpoint_prefix
the prefix of a V1 or V2 checkpoint, with V2 taking
priority. Typically the result of Saver.save() or that of
tf.train.latest_checkpoint(), regardless of sharded/non-sharded or
V1/V2.
Returns
A bool, true if a checkpoint referred to by checkpoint_prefix exists.