[backport] Fix decorrelated batch normalization when groups ≠ 1

[hvy]

Backport of #7707

[hvy]

[automatic post] Jenkins, test this please.

[hvy]

We need to backport this PR first #7226 ?

[toslunar]

#7226

chainer.backend.copyto is relevant but Variable.copydata is not.

[chainer-ci]

Jenkins CI test (for commit e532541, target branch v6) failed with status FAILURE.

[toslunar]

I've not understood the difference between

chainer/chainer/serializers/npz.py

Lines 180 to 182 in 397fb99

	if isinstance(value, chainerx.ndarray):
	value_view = chainerx.to_numpy(value, copy=False)
	numpy.copyto(value_view, dataset)

and

chainer/chainer/backend.py

Lines 53 to 54 in 397fb99

	if isinstance(dst, chainerx.ndarray):
	dst[...] = _chainerx._array_to_chainerx(src, dst.device)

[hvy]

The former indeed seems broken #7829

[niboshi]

TODO: Backport #7828 after this.

[hvy]

chainer.backend.copyto is relevant but Variable.copydata is not.

@toslunar Are there concerns with backporting #7226 ? I'd like to do so otherwise.

[niboshi]

ping

[toslunar]

I made #7934. Whether to backport the other part of #7226 might be discussed later.

[niboshi]

#7934 merged.

[hvy]

@toslunar could you take another look at this backport now that your PR has been merged?

[toslunar]

Jenkins & flexCI, test this please.

[chainer-ci]

Jenkins CI test (for commit 7d8cf58, target branch v6) succeeded!

[toslunar]

LGTM