ndarray: support modulo operation in macro inplace (#390)

* ndarray: inplace modulo operators

```julia
mod_from!(x, y)
mod_from!(x, 2)
rmod_from!(2, x)
```

* ndarray: support modulo operation for macro inplace

Blocker: #389

* update doc

NEWS.md

@@ -7,6 +7,7 @@
         * `context()`
         * `empty()`
         * `expand_dims()`
+        * `@inplace`
         * `σ()`
         * `sigmoid()`
         * `relu()`

docs/src/api/ndarray.md

@@ -11,6 +11,7 @@ In the following example `y` can be a `Real` value or another `NDArray`
 | `*` | `x .* y` | Elementwise multiplication |
 | `/` | `x ./ y` | Elementwise division       |
 | `^` | `x .^ y` | Elementwise power          |
+| `%` | `x .% y` | Elementwise modulo         |
 
 
 ## Trigonometric Functions

src/MXNet.jl

@@ -36,6 +36,7 @@ export NDArray,
        context,
        empty,
        expand_dims,
+       @inplace,
        # activation funcs
        σ,
        sigmoid,

src/ndarray.jl

@@ -572,18 +572,21 @@ will translate into
 
 which will do inplace adding of the contents of `b` into `a`.
 """
-macro inplace(stmt)
-  if stmt.head == :+= || stmt.head == :.+=
-    Expr(:call, :add_to!, esc(stmt.args[1]), esc(stmt.args[2]))
-  elseif stmt.head == :-= || stmt.head == :.-=
-    Expr(:call, :sub_from!, esc(stmt.args[1]), esc(stmt.args[2]))
-  elseif stmt.head == :.*=
-    Expr(:call, :mul_to!, esc(stmt.args[1]), esc(stmt.args[2]))
-  elseif stmt.head == :./=
-    Expr(:call, :div_from!, esc(stmt.args[1]), esc(stmt.args[2]))
+macro inplace(ex)
+  f = if ex.head == :+= || ex.head == :.+=
+    :add_to!
+  elseif ex.head == :-= || ex.head == :.-=
+    :sub_from!
+  elseif ex.head == :.*=
+    :mul_to!
+  elseif ex.head == :./=
+    :div_from!
+  elseif ex.head == :.%=
+    :mod_from!
   else
-    error("unsupported inplace translation for $stmt")
+    error("unsupported inplace translation for $ex")
   end
+  Expr(:call, f, esc(ex.args[1]), esc(ex.args[2]))
 end
 
 """

test/unittest/ndarray.jl

@@ -594,6 +594,15 @@ function test_mod()
     @test_throws AssertionError mx.mod_from!(x, 2)
     @test_throws AssertionError mx.rmod_from!(2, x)
   end
+
+  info("NDArray::mod::inplace")
+  let
+    x = NDArray(A)
+    y = NDArray(B)
+    C = A .% B
+    @inplace x .%= y
+    @test copy(x) ≈ C
+  end
 end  # function test_mod