common_shape = [1] * reduce(max, (len(shape) for shape in shapes))
for arg_idx, shape in enumerate(shapes):
    for idx in range(-1, -1 - len(shape), -1):          # align from the right
        if common_shape[idx] == 1:
            common_shape[idx] = shape[idx]              # 1 stretches to anything
        if shape[idx] == 1:
            continue                                    # ... in either direction
        torch._check(common_shape[idx] == shape[idx],   # otherwise sizes must match
            lambda: f"Attempting to broadcast a dimension of length {shape[idx]} at {idx}!")
return common_shape