ndarray: support transpose on 1D array (#375)

Python doesn't have this functionality,
so I implement it via `reshape`.

```julia
julia> x = NDArray(Float32[1, 2, 3, 4])
4 mx.NDArray{Float32,1} @ CPU0:
 1.0
 2.0
 3.0
 4.0

julia> x'
1×4 mx.NDArray{Float32,2} @ CPU0:
 1.0  2.0  3.0  4.0
```

NEWS.md

@@ -107,6 +107,21 @@
    4.0
   ```
 
+* Transposing a column `NDArray` to a row `NDArray` is supported now. (#TBD)
+
+  ```julia
+  julia> x = NDArray(Float32[1, 2, 3, 4])
+  4 mx.NDArray{Float32,1} @ CPU0:
+   1.0
+   2.0
+   3.0
+   4.0
+
+  julia> x'
+  1×4 mx.NDArray{Float32,2} @ CPU0:
+   1.0  2.0  3.0  4.0
+  ```
+
 ## API Changes
 
 ### `NDArray`

src/ndarray.jl

@@ -1114,7 +1114,8 @@ _mxsig[:reshape] = :(reshape(arr; shape = dim, reverse = !reverse))
 @_remap dot(x::NDArray{T,N}, y::NDArray{S,N}) where {T,S,N} dot(y, x)
 
 # See https://github.com/dmlc/MXNet.jl/pull/123
-@_remap transpose(arr::NDArray) transpose(_only2d(arr))
+@_remap transpose(arr::NDArray{T,1}) where T reshape(arr; shape = (1, length(arr)), reverse = true)
+@_remap transpose(arr::NDArray{T,2}) where T transpose(arr)
 @_remap permutedims(arr::NDArray, axes) transpose(arr; axes = length(axes) .- tuple(axes...))
 
 @_remap prod(arr::NDArray)       prod(arr)

src/util.jl

@@ -163,11 +163,6 @@ function _format_signature(narg::Int, arg_names::Ref{char_pp})
   return join([unsafe_string(name) for name in arg_names] , ", ")
 end
 
-@inline function _only2d(x)
-  @assert ndims(x) == 2
-  x
-end
-
 """
 libmxnet operators signature checker.
 

test/unittest/ndarray.jl

@@ -775,7 +775,13 @@ function test_fill()
 end  # function test_fill
 
 function test_transpose()
-  info("NDArray::transpose")
+  info("NDArray::transpose::1D")
+  let A = rand(Float32, 4), x = NDArray(A)
+    @test size(x) == (4,)
+    @test size(x') == (1, 4)
+  end
+
+  info("NDArray::transpose::2D")
   let A = rand(Float32, 2, 3), x = mx.NDArray(A)
     @test size(x) == (2, 3)
     @test size(x') == (3, 2)