Fix vstack for mixed fixed-shape inputs#2897

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xtensor-stack:masterxtensor-stack/xtensor:masterfrom

Alex-PLACET:add-vstack-functionality-for-fixed-shape-tensors-in-xbuilderAlex-PLACET/xtensor:add-vstack-functionality-for-fixed-shape-tensors-in-xbuilderCopy head branch name to clipboard