Fix array indexing in create_multi_node_evaluator

[emcastillo]

Closes #8546

In some cases local_mean_dict.values() is empty. For example in the above issue, it is empty for the rank 1 since the example explicitly creates an empty dataset for this rank.

chainer/examples/chainermn/mnist/train_mnist_dual_parallel.py

Lines 124 to 126 in 8b9e9c0

	if data_axis == 1:
	train = chainermn.datasets.create_empty_dataset(train)
	test = chainermn.datasets.create_empty_dataset(test)

Also metrics are not reported for rank 1.

chainer/examples/chainermn/mnist/train_mnist_dual_parallel.py

Lines 107 to 110 in 8b9e9c0

	if data_axis == 0:
	model = L.Classifier(MLP0(model_comm, args.unit))
	elif data_axis == 1:
	model = MLP1(model_comm, args.unit, 10)

[keisukefukuda]

pfnci, test this please.

[chainer-ci]

Jenkins CI test (for commit 813a518, target branch master) succeeded!

[keisukefukuda]

Test are failing. This does not seem to be related to this PR, but could you take a look at it?

[emcastillo]

Yes, tests has been failing in PfnCI for a while due to out of memory errors,
We have to fix this 🙇‍♂️.
Jenkins worked fine and the errors are not related to the PR so I think it is safe to merge.

Sorry and thank you again.

[emcastillo]

All the CI issues were fixed, the current failing job is due to having the env variables set on job creation so it can't be overridden with the new values unless the CIs are kicked again ☹️

[kmaehashi]

pfnCI, test this please.

[chainer-ci]

Jenkins CI test (for commit 813a518, target branch master) failed with status FAILURE.

[emcastillo]

Jenkins, test this please

[chainer-ci]

Jenkins CI test (for commit 813a518, target branch master) succeeded!

[emcastillo]

@keisukefukuda we fixed the CIs and now all test passed :D