Rename TShape::index_t to dim_t and change to int64_t (#114)

* Change TShape::index_t to int64_t

* Add comment

* Make Tuple::Save&Load dtype generic

* trigger update

* Fix lint

* Fix comment

* Change index_t to dim_t

* Remove legacy index_t

nnvm/src/pass/gradient.cc

@@ -156,7 +156,7 @@ Graph Gradient(Graph src) {
         CHECK_EQ((*rit)->inputs.size(), input_grads.size())
             << "Gradient function not returning enough gradient";
       } else if (CheckGradAllZero(out_agg_grads, zero_ops)) {
-        for (index_t i = 0; i < fwd_node->num_inputs(); ++i) {
+        for (size_t i = 0; i < fwd_node->num_inputs(); ++i) {
           std::ostringstream os;
           if (1 == fwd_node->num_inputs()) {
             os << fwd_node->attrs.name << "_backward";

nnvm/tests/cpp/tuple_test.cc

@@ -17,7 +17,7 @@ TEST(Tuple, Basic) {
   std::istringstream is(os.str());
   is >> y;
   CHECK_EQ(x, y);
-  Tuple<nnvm::index_t> ss{1, 2, 3};
+  Tuple<nnvm::dim_t> ss{1, 2, 3};
   TShape s = ss;
   s = std::move(ss);
   CHECK((s == TShape{1, 2, 3}));