Implementation in CuPy with `stream=` and `dl_device=cpu` choices

[seberg]

I implemented dlpack v1 for CuPy (see https://github.com/cupy/cupy/pull/8683), and there are two choices that are important for other implementations and maybe the spec:

1. We chose to export the cudaManaged device when possible even if dl_device=(CPU, 0) was requested. I.e. we promise that the data can be used on the CPU device, but cupy currently will still give you the actual (compatible) device!
   • Note: NumPy is OK with this in the case of cuda managed memory. But it may not yet be OK with it in the case of future/other similar devices. (I.e. NumPy may need to trust the producer in this case, or we just keep it a bit of a fuzzy thing where we assume the consumer should know the device, possible based on version.)
2. If user passes dl_device=(CPU, 0), stream=.... We had discussed that the semantics must be related to the device that the data is on, I think. CuPy supports this:
   • stream=None (or nothing passed), will synchronize the device to host copy (i.e. wait until the data is CPU available).
   • stream=consumer_stream will not synchronize. The user could in theory work with the data (e.g. another cudaAsyncCopy) on consumer_stream, or synchronize themselves (e.g. if multiple copies needed).
   • REASON: One reason is that synchronizing in the second case would achieve nothing that stream=None doesn't already achieve. It would effectively do the same stream=None and also synchronize the consumer_stream. (But that stream does not need to be synchronized!)

CC @leofang.