Fix issues with _CPackedColumns.serialize() handling of host and device data

[charlesbluca]

A few changes in here to resolve some test failures using pack/unpack with DataFrame serialization:

• We now use a Buffer in frames to represent the device data, so that Dask can correctly perform the necessary DtoD transfers when moving a packed columns object between devices
• With Fix a crash in pack() when being handed tables with no columns. #8697 merged, the results of packing an empty DataFrame will set the pointer to the host data to NULL; since Cython cannot make a memoryview from NULL, we now check for this condition before making the host data array
• The serialized type is now included in the serialized header

[isVoid]

Small Q

[isVoid]

Would it be better to directly call Buffer.serialize() (and subsequently Buffer.deserialize() below) ?

[charlesbluca]

I think we need to leave this as a Buffer so we are able to inherit the additional methods of Serializable; see this assertation in Serializable.device_serialize():

cudf/python/cudf/cudf/core/abc.py

Lines 98 to 100 in 5b8895d

	assert all(
	(type(f) in [cudf.core.buffer.Buffer, memoryview]) for f in frames
	)

[isVoid]

Sorry if this is a dumb question, could you elaborate: why Buffer.deserialize() fails to inherit the additional methods?

[charlesbluca]

Ah my mistake I was thinking of just passing the output of Buffer.deserialize() into frames, which wouldn't work because of the assertion above - you are correct, we should be serializing the Buffer here, and add the resulting header to the PackedColumns header.

[codecov]

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[charlesbluca]

@gpucibot merge