!106 GPU update cast

Merge pull request !106 from VectorSL/gpu-cast

python/akg/ms/gpu/cast.py

@@ -22,4 +22,6 @@ def Cast(x, dst_type):
     """cast."""
     if x.dtype == "int64" and dst_type == "float16":
         x = cast.cast(x, "float32")
+    if x.dtype == "float16" and dst_type == "int32":
+        x = topi.trunc(x)
     return cast.cast(x, dst_type)