* bugfix for update with empty buffer; remove duplicate variable _weight_sum in PrioritizedReplayBuffer
* point out that ListReplayBuffer cannot be sampled
* remove useless _amortization_counter variable
* Use lower-level API to reduce overhead.
* Further improvements.
* Buffer _add_to_buffer improvement.
* Do not use _data field to store Batch data to avoid overhead. Add back _meta field in Buffer.
* Restore metadata attribute to store batch in Buffer.
* Move out nested methods.
* Update try/catch instead of actual check to efficiency.
* Remove unsed branches for efficiency.
* Use np.array over list when possible for efficiency.
* Final performance improvement.
* Add unit tests for Batch size method.
* Add missing stack unit tests.
* Enforce Buffer initialization to zero.
Co-authored-by: Alexis Duburcq <alexis.duburcq@wandercraft.eu>
