Minor degenerate edge case when resuming training

[kttian]

If we run training resuming from epoch N and max epoch is <= N, then we don’t enter the train loop, so cur_epoch is not defined at line 409, causing an error (https://github.com/salesforce/LAVIS/blob/main/lavis/runners/runner_base.py#L409). I guess the user should not be specifying hyperparameters like this, but I thought I would flag it.