Change TF 2.0 tf.summary.SummaryWriter to map 1:1 to underlying resource

This changes the resource model for the TF 2.0 tf.summary.SummaryWriter as returned by tf.summary.create_file_writer().  Previously, we preserved the 1.x tf.contrib.summary.create_file_writer() semantics where underlying resources were shared by logdir to facilitate reuse of a single eventfile per directory, which avoids accidentally overwriting an existing eventfile (which is possible if both are created within the same second) and is better supported by TensorBoard. Unfortunately, that sharing behavior resulted in buggy and confusing resource deletion behavior as described in #25707, and is also out of alignment with the overall TF 2.0 philosophy that resource lifetimes should match the lifetimes of their encapsulating Python objects.

The new SummaryWriter wrapper object fixes this by generating a unique resource name for each instance. Inside @tf.functions, this requires lifting initialization out using init_scope(), which means in-graph tensors can no longer be passed to create_file_writer(), and the returned SummaryWriter must be stored in a non-local variable so that it outlives the tracing phase and is still alive when the function graph executes. A future change will add better error messaging for the latter circumstance.

To keep the SummaryWriter <-> resource mapping 1:1, this also changes the close() method so that in eager mode it will permanently close the SummaryWriter and prevent re-initialization. (A future change may do this for graph mode as well.)

To mitigate the risk of eventfile name collisions, the new wrapper bakes the PID and a TF-generated UID into the filename suffix.

Finally, this cleans up the public API surface by 1) moving the "logdir=None means no-op" behavior into a dedicated create_noop_writer() factory function and 2) hiding SummaryWriter.__init__() from the public API by making SummaryWriter an abstract base class; instances should be obtained via factory functions anyway.

Fixes #25707. A subsequent change will update the TF 2.0 Keras callback to call create_file_writer_v2(), which should address #25524 and #24632.

PiperOrigin-RevId: 235249883