DPPLQueue_Memcpy should return an Event handle and not wait.

[diptorupd]

Currently DPPLQueueMemcpy waits on the event returned by sycl::queue.memcpy before returning. We should instead just return the event back to caller.

[diptorupd]

@adarshyoga I am assigning this ticket to you as it will be related to the asynchronous kernel execution improvements you are working on for Numba-dppy.

[diptorupd]

@adarshyoga ping.

[diptorupd]

@oleksandr-pavlyk I have started making the changes needed to fix the issue. However, I wanted to clarify few things that will be impacted by making DPCTLQueue_Memcpy non-blocking.

1. The SyclQueue.memcpy function only allows copying from one _Memory object to another. However, what is the expected behavior when the _Memory objects are not using the same queue as the SyclQueue instance on which memcpy is invoked?
2. Similar to (1), but the _Memory objects are using two different queue.
3. Why should the memcpy functions be restricted to just between two _Memory objects?
4. The copy* functions in _Memory all use DPCTLQueue_Memcpy. Should we change these functions to non-blocking as well?
5. Should we introduce the dependent events arguments to the functions at this point?

[diptorupd]

fixed in #557