Several bug fix and enhancement:

• remove deprecated API append (#126)
• Batch.cat_ and Batch.stack_ is now working well with inconsistent keys (#130)
• Batch.is_empty now correctly recognizes empty over empty Batch (#128)
• reconstruct collector: remove multiple buffer case, change the internal data to Batch, and add reward_metric for MARL usage (#125)
• add Batch.update to mimic dict.update (#128)