fix definition of GeneralDimensions structure

Author: ytdHuang

src/qobj/dimensions.jl

@@ -35,14 +35,14 @@ Dimensions(dims::Any) = throw(
 )
 
 @doc raw"""
-    struct GeneralDimensions{N,T1,T2} <: AbstractDimensions{N}
+    struct GeneralDimensions{N,T1<:Tuple,T2<:Tuple} <: AbstractDimensions{N}
         to::T1
         from::T2
     end
 
 A structure that describes the left-hand side (`to`) and right-hand side (`from`) Hilbert [`Space`](@ref) of an [`Operator`](@ref).
 """
-struct GeneralDimensions{N,T1,T2} <: AbstractDimensions{N}
+struct GeneralDimensions{N,T1<:Tuple,T2<:Tuple} <: AbstractDimensions{N}
     # note that the number `N` should be the same for both `to` and `from`
     to::T1   # space acting on the left
     from::T2 # space acting on the right