[RUNTIME] Ensure NDArray.CopyTo(Device) always sync #16716
Merged
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This PR ensures that NDArray.CopyTo(Device) always sync.
Prior to this PR, some of the behavior relied on undefined behavior, as the per convention the underlying DeviceAPI may be async, but due to the cuda implementations was sync, we implicitly relied on that behavior.
This PR further clarifies in docs about the contract (that low-level device api is always async) as well as the sync/async nature of each NDArray API.