fix: skip aio wait when swap tensors is empty (#7712)
This PR fixes https://github.com/deepspeedai/DeepSpeed/issues/7703.
The root cause is that when `swap_tensors` is empty, no AIO request is
queued. However, `aio.wait()` still checks whether any AIO request has
been issued, which can lead to `_num_pending_ops == 0` and trigger the
issue:
https://github.com/deepspeedai/DeepSpeed/blob/c069ceb30c2eca0380f65338f04693d9586e09cb/csrc/aio/py_lib/deepspeed_py_io_handle.cpp#L191
The fix is to skip `aio_handle.wait()` when there are no swap tensors
queued.
