Add tf.summary.write and tf.summary.summary_scope for TF 2.0

The write() op is the TF 2.0 version of tf.contrib.summary.generic() and still backed by the WriteSummary C++ op, but with some changes:
- first parameter is now the full summary tag, instead of deriving tag from "name"
- step parameter is now mandatory since there's no global step
- recording is "on by default" - no longer requires always_record_summaries()
- no more "family" parameter - this should be done via proper scoping

The summary_scope() is a helper for writing TF 2.0 summary ops like scalar(), etc. that provides better semantics for how tags are generated relative to name scopes.

PiperOrigin-RevId: 225414160