fixed the order of arguments of _update_mems() function. No impact fu…

[victor-psiori]

Fixed the order of arguments for the _update_mems() function of MemTransformerLM class.

• Earlier at line: https://github.com/kimiyoung/transformer-xl/blob/44781ed21dbaec88b280f74d9ae2877f52b492a5/pytorch/mem_transformer.py#L733 , arguments were passed in order: hids, mems, mlen, qlen whereas class function definition expects in order: hids, mems, qlen mlen.
• No functional impact though since qlen and mlen are added together for calculating context token length.
• Rest of the changes caused by Black python formatter and could be ignored.

Cheers, victor