CPU-only unit tests for :mod:fastvideo.train.callbacks.ema.
Exercises the EMA lifecycle (lazy init, start_iter gating, decay
math, ema_context swap, state-dict round-trip) on a tiny CPU
nn.Linear. EMA_FSDP works without dist.init_process_group
because dist.is_initialized() returns False and _to_local_tensor
falls through to raw tensors for non-DTensor inputs.