Add n_generate and save_gdrive variables to ATGProgressCallback

[Meorge]

I started trying to use the trainer branch of aitextgen, and found that it crashed when attempting to either save the model or generate texts from it while training, due to the n_generate and save_gdrive variables not having been created. This pull request adds those variables as members of ATGProgressCallback so that training can complete.

[Meorge]

Oops, sorry about that. I didn't realize that all changed pushed to a branch would be included in the pull request! 😅 Just fixed it so that this pull request only incorporates the changes described in the original post.