When resuming training from checkpoint, Trainer loads model
# What does this PR do?

Trainer was not reloading model when resuming training from a checkpoint, which was confusing for users (see #9099) and also was preventing the recent auto-reload from checkpoint to fully work.

This isn't a breaking change (if users were passing a model with the checkpoint already loaded, it is just loaded twice).
If I may add, under `def train():`, I think the initialisation of `self._globalstep_last_logged = 0` should be `self._globalstep_last_logged=self.state.global_step`, to ensure that the first logging of the loss is correct when you later divide by `self.state.global_step-self._globalstep_last_logged`? 