- Load schedulers from ckpt in a backward-compatible manner

[hyperzy]

• Allow loading old checkpoints without the schedulers key.

[brentyi]

Can we leave a comment explaining these conditions? (ideally with some info on what versions of nerfstudio this impacts)

[hyperzy]

Versions <= 0.3.2 don't support loading schedulers, because the states of schedulers are not saved in Trainer.save_checkpoint(). Specifically, commits older than cebe787 don't support schedulers loading/saving.
These changes in the commit will save the states of schedulers by default, i.e. creating a schedulers key. Were the checkpoints from previous versions loaded, the loader will check whether the schedulers key exist before loading schedulers.