Bunch of fixes.

Fixed a few essential bugs.

• (cache_until mechanism): The roles of waiting nodes and notifier nodes
  in cache_until mechanism are in reverse. Fixed.
• (Event sources resetting): Event sources are not correctly reset during
  task resetting. Fixed.
• (Events resetting): Events may be triggered mistakenly by the stale
  emulator status owing to the delay of emulator. Fixed.

Fixed minor bugs.

• (Requirements of Gymnasium and dm-env): Gymnasium and dm-env are marked
  as optional dependencies, however, they are imported in the wrappers module
  as required sections. Fixed.
• (Crash when meeting unseen special tokens): The old behavior will crash
  when meeting special tokens from the tokenizer. Now the new behavior is to
  ignore them directly.

Migrated logging system from absl to the embedded logging module.
Updated the template toolkit to enable non-automatic concatenation of
sub-task commands.