Create FileWriter <-> tf.contrib.summary compatibility layer

This provides an implementation of FileWriter, activated by passing in a `session` parameter to the constructor, that is backed by session.run'ing graph ops that manipulate a tf.contrib.summary.create_file_writer() instance. Because tf.contrib.summary.SummaryWriters are backed by shared resources in the graph, this makes it possible to have a FileWriter and a tf.contrib.summary.SummaryWriter that both write to the same events file.

This change includes some related smaller changes:
- Factors out training_utils.py into a separate target to avoid a cyclic dep
- Moves contrib/summary/summary_ops.py to python/ops/summary_ops_v2.py
- Adds SummaryWriter.init(), .flush(), and .close() op-returning methods
- Changes create_file_writer() `name` arg to default to logdir prefixed by `logdir:` so shared resources are scoped by logdir by default
- Fixes a bug with tf.contrib.summary.flush() `writer` arg
- Makes create_file_writer()'s max_queue arg behave as documented
- Adds more testing for existing tf.contrib.summary API

PiperOrigin-RevId: 192408079