[Bug] Fix that pin_prefetcher is not actually enabled

[yaox12]

Description

Fix that pin_prefetcher is not actually enabled.

Changes

1. Set pin_memory=self.pin_prefetcher to enable pin memory for returned indices. Note: subgraph is still not pinned.
2. Correctly enable pin memory for column fetching.

[yaox12]

I think pin_prefetcher now has the same semantic as Pytorch dataloader's pin_memory except that sampled subgraphs are not pinned yet. pin_prefetcher is valid only when sampling is on CPU and destination device is GPU and not use_uva, so pinning CUDA tensors won't occur.

dgl/python/dgl/dataloading/dataloader.py

Lines 735 to 743 in ec74015

	# (BarclayII) I hoped that pin_prefetcher can be merged into PyTorch's native
	# pin_memory argument. But our neighbor samplers and subgraph samplers
	# return indices, which could be CUDA tensors (e.g. during UVA sampling)
	# hence cannot be pinned. PyTorch's native pin memory thread does not ignore
	# CUDA tensors when pinning and will crash. To enable pin memory for prefetching
	# features and disable pin memory for sampler's return value, I had to use
	# a different argument. Of course I could change the meaning of pin_memory
	# to pinning prefetched features and disable pin memory for sampler's returns
	# no matter what, but I doubt if it's reasonable.

And since the previous pin_prefetcher didn't take effect correctly (as fixed in this PR), I think merging pin_prefetcher into PyTorch's native pin_memory would not affect existing code and examples.

[dgl-bot]

To trigger regression tests:

• @dgl-bot run [instance-type] [which tests] [compare-with-branch];
  For example: @dgl-bot run g4dn.4xlarge all dmlc/master or @dgl-bot run c5.9xlarge kernel,api dmlc/master

[dgl-bot]

Commit ID: 62c39e3

Build ID: 2

Status: ✅ CI test succeeded

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