Add APIs for creating NumPy arrays backed by pinned memory

[leofang]

Close #3625.

This PR adds a few convenience functions to the cupyx namespace for allocating NumPy arrays backed by pinned/pagelocked memory, see the discussion around #3625 (comment). Currently the provided APIs are as follows:

• cupyx.empty_pinned()
• cupyx.empty_like_pinned()
• cupyx.zeros_pinned()
• cupyx.zeros_like_pinned()

with exactly the semantics as their NumPy counterparts.

Ideally, we can do better than this if we use NumPy's memory hooks to always allocate NumPy arrays from pinned memory (numpy/numpy#17467), however it's still a WIP, so we can only revisit it when it's in.

TODO:

• Decide if the name/API is appropriate or not
• Add docstrings
• Fix order = 'K'
• Add tests

[leofang]

Help needed: I am writing tests and I noticed @testing.numpy_cupy_array_equal() does not work for this PR, because we are comparing two numpy.ndarrays which breaks the assumption of this test utility. Any advice?

So I wrote a decorator @numpy_cupyx_array_equal() to address this need. The tests added in the new tests/cupyx_tests/test_pinned_array.py largely follows tests/cupy_tests/creation_tests/test_basic.py for the four added functions, with this decorator change and some other necessary but minor changes.

[leofang]

Flake8 error will be fixed in #4888.

[leofang]

Jenkins, test this please

[chainer-ci]

Jenkins CI test (for commit e5f357b, target branch master) failed with status FAILURE.

[leofang]

Jenkins, test this please

[chainer-ci]

Jenkins CI test (for commit 31d2338, target branch master) succeeded!

[chainer-ci]

Jenkins CI test (for commit c6804a8, target branch master) succeeded!

[leofang]

@kmaehashi Errors are due to OSError: [Errno 28] No space left on device.

[kmaehashi]

I've freed up some space now.

pfnCI, test this please.

[chainer-ci]

Jenkins CI test (for commit c6804a8, target branch master) succeeded!

[leofang]

hmmm...I wonder why Jenkins still shows a red cross despite chainer ci says success?

[kmaehashi]

It seems Jenkins github plugin was using deprecated github APIs and now they're removed. I bumped the plugin version so it should work now.

[jakirkham]

Jenkins, test this please

[chainer-ci]

Jenkins CI test (for commit c6804a8, target branch master) succeeded!

[leofang]

Resolved conflicts.

Jenkins, test this please

[chainer-ci]

Jenkins CI test (for commit 500d53c, target branch master) failed with status FAILURE.

[leofang]

Jenkins, test this please

[chainer-ci]

Jenkins CI test (for commit cc6fe6c, target branch master) succeeded!

[leofang]

@kmaehashi as mentioned I'd love to have this PR backported 🙏

[leofang]

Jenkins, test this please

[kmaehashi]

LGTM!

[chainer-ci]

Jenkins CI test (for commit cc6fe6c, target branch master) succeeded!

[leofang]

Thanks, @kmaehashi and @jakirkham!