Fix StatefulDataLoader KeyError with num_workers > 0 (#3931)
When num_workers > 0, StatefulDataLoader uses
_MultiProcessingDataLoaderIter which produces a different state_dict
structure that may not contain _sampler_iter_yielded or _num_yielded
keys. The adjust_state_dict_for_prefetch method was accessing these
keys directly, causing a KeyError.
Added key existence checks before accessing these state_dict entries.
Fixes #3110
