Two unrelated features on top my wishlist would solve most of your issue as a byproduct:
Replacing variadics ([Discussion] Variadics as an Attribute) and constants as generic parameters (Proposal: Compile-time parameters).
With those, your signature would be
func mean(_ alongAxes: @variadic MinimalSizeArray<Int, 1>) -> Tensor
I think it's better than adding yet another special case (even if it takes longer to be implemented).