change img.stack to img.concatenate of loadmultichannelimagefromfile

[ggamaz]

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Motivation

when the read images's shape is [h, w, c], stack make the result shape is [h, w, c, n], and concatenate result is [h, w, n*c].
when the read images's channel is not same, stack raise an error, may concatenate is more suit for LoadMultiChannelImageFromFiles

Modification

change np.stack(img, -1) to np.concatenate(img, -1)

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMPreTrain.
4. The documentation has been modified accordingly, like docstring or example tutorials.

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