fix bugs with trainer
# What does this PR do?
Context:
1. Issue 1 - Currently, when the lr_scheduler is specified in the deepspeed config file, we leverage a DummyScheduler to pass to the `accelerator.prepare` to get the correct scheduler post prepare call. A prior PR removed preparation of the lr_scheduler leading to a lot of DeepSpeed tests failing.
2. Issue 2 - when using apex we shouldn't be preparing optimizer else we get `AttributeError: 'AcceleratedOptimizer' object has no attribute '_amp_stash'`
3. Issue 3 - FSDP ckpt logic should create ckpt dir if not present. Fixes https://github.com/huggingface/transformers/issues/24130

This PR fixes the above issues. 
_The documentation is not available anymore as the PR was closed or merged._