ndarray: add Base.show(io, MIME"text/plain") (#347)

* ndarray: add `Base.show(io, MIME"text/plain")`

e.g. make Array of NDArray show like this
```julia
julia> [mx.zeros(100)]
1-element Array{MXNet.mx.NDArray,1}:
 NDArray Float32[0.0, 0.0, 0.0  … 0.0, 0.0, 0.0]
```

* test cases

src/ndarray.jl

@@ -105,9 +105,15 @@ const NDArrayOrReal = Union{NDArray, Real}
 
 @unfuse NDArray
 
-function Base.show(io :: IO, arr :: NDArray)
-  println(io, "$(join(size(arr), "×")) mx.NDArray{$(eltype(arr))} @ $(context(arr)):")
-  Base.showarray(io, try_get_shared(arr, sync=:read), false, header=false)
+function Base.show(io::IO, x::NDArray)
+  print(io, "NDArray ")
+  Base.showarray(io, try_get_shared(x, sync = :read), header = false)
+end
+
+# for REPL
+function Base.show(io::IO, ::MIME{Symbol("text/plain")}, x::NDArray)
+  println(io, "$(join(size(x), "×")) mx.NDArray{$(eltype(x))} @ $(context(x)):")
+  Base.showarray(io, try_get_shared(x, sync = :read), false, header = false)
 end
 
 function Base.unsafe_convert(::Type{MX_handle}, obj::NDArray)

test/unittest/ndarray.jl

@@ -741,13 +741,23 @@ function test_transpose()
 end
 
 function test_show()
-  let str = sprint(show, mx.NDArray([1 2 3 4]))
+  info("NDArray::show::REPL")
+  let str = sprint(show, MIME"text/plain"(), mx.NDArray([1 2 3 4]))
     @test contains(str, "1×4")
     @test contains(str, "mx.NDArray")
     @test contains(str, "Int64")
     @test contains(str, "CPU")
     @test match(r"1\s+2\s+3\s+4", str) != nothing
   end
+
+  info("NDArray::show")
+  let str = sprint(show, mx.NDArray([1 2 3 4]))
+    @test str == "NDArray [1 2 3 4]"
+  end
+
+  let str = sprint(show, mx.zeros(4))
+    @test str == "NDArray Float32[0.0, 0.0, 0.0, 0.0]"
+  end
 end
 
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