Fix vstack for mixed fixed-shape inputs#2897

Alex-PLACET wants to merge 3 commits intoxtensor-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