Reduce usage of tf.compat.v1 APIs

[stephanwlee]

TensorBoard have migrated to TF 2.0 APIs but there are some usages of V1 APIs due to not-so-compatible changes. Since we are interested in moving off of tf.compat.v1, we will either have to find a replacement or introduce some changes in behavior.

Do note that this list exclude v1 API usages in tests (they are mainly written in graph style and is tracked in #1705) and TB v1 summary.

• Converted tf.app.run to absl.app.run #1725: tf.compat.v1.app -> absl.app
• Use absl.flags instead of tf.flags #1726: tf.compat.v1.flags -> absl.flags
• Use v2 API for GFile #1733: tf.compat.v1.gfile.GFile -> io.gfile.GFile
• Use v2 API for GFile #1733: tf.compat.v1.gfile.Open -> io.gfile.GFile
• Use the renamed image.resize TF API #1734: tf.compat.v1.image.resize_images -> tf.image.resize
• Use the renamed image.resize TF API #1734: tf.compat.v1.image.resize_nearest_neighbor -> tf.image.resize
• Change TF graph proto to TB graph proto #1730: tf.compat.v1.GraphDef -> TB proto
• Change TF graph proto to TB graph proto #1730: tf.compat.v1.MetaGraphDef -> TB proto
• [CLEANUP] tf.RunMetadata to tb.RunMetadata #1721: tf.compat.v1.RunMetadata -> TB proto
• [CLEANUP] Change tf.SessionLog.* to TB.SessionLog.* #1720: tf.compat.v1.SessionLog -> TB proto
• Update TensorBoard to import pywrap_tensorflow from tensorflow.python #2096: tf.compat.v1.pywrap_tensorflow.NewCheckpointReader -> tf.train.load_checkpoint
• replace deprecated tf.compat.v1.train.checkpoint_exists() #2157: tf.compat.v1.checkpoint_exists -> tf.io.gfile.glob() for the prefix
• tf.compat.v1.Session -> Use eager mode
• tf.contrib.ffmpeg -> tf.audio.encode_wav or python stdlib's wave module
• tf.contrib.summary.create_db_writer -> replace with python implementation (obsolete)
• tf.compat.v1.pywrap_tensorflow.EventsWriter -> migrated to tensorflow.python._pywrap_events_writer private API via Update TensorBoard to import pywrap_tensorflow from tensorflow.python #2096 and then Use _pywrap_events_writer #2683.
• tf.compat.v1.io.tf_record_iterator -> tracked in Replace tf_record_iterator with a non-deprecated way to read TFRecords #1711
• tf.compat.v1.train.summary_iterator - should become our own wrapper of tf_record_iterator for the places we need it (only tests)
• Change EventFileLoader to use tf_record_iterator when possible #3185: tf.compat.v1.pywrap_tensorflow.PyRecordReader_New -> converted to be only a legacy fallback for when we can't use tf.compat.v1.io.tf_record_iterator

[nfelt]

Edited to note that I think tf.compat.v1.train.summary_iterator should also be tracked in #1711. tf.train.summary_iterator should go away in TF 2.0, and we should replace it with whatever we resolve #1711 with.