v0.12.8

Added

• #v1 Add logger parameter to v1.training.Checkpointer and log
  RestoreStepStatistics in load_checkpointables.
• #v1 Add delete and delete_async free functions and
  training.Checkpointer.delete / delete_async for deleting an entire
  checkpoint or a single named checkpointable.
• #v1 Add regulator_options (MemoryOptions.RegulatorOptions) and
  expected_surge_bytes to v1.options.MemoryOptions for dynamic D2H transfer
  memory regulation.

Fixed

• Consolidate GCS kvstore spec generation and support gcs_grpc with OCDBT
  kvstack.
• Fix CheckpointManager.wait_until_finished() to only accumulate
  wait_for_prev_duration_secs when called from the main thread.