Add missing flags for pretrain experiment (AttributeError)

[Maxvgrad]

Fixing error an AttributeError for the restrict_attention_bucket_size flag is raised when running the tapas_pretraining_experiment script.

Explicitly providing the command line flag doesn't solve the the problem (Unknown command line flag 'restrict_attention_bucket_size'). Probably, the flag was not defined in the script file during changes related to MATE

Resolve the issue: https://github.com/google-research/tapas/issues/184