[NODE][Serialization]fix serialization precision loss in float (apach…

…e#4503)

* fix serialization precision loss in float

When we want to serialize a tvm.tensor object(like pickle)， we will get a precision loss cause by std::to_string()。
For example, a2.value will be 0.0 while a.value=0.00000001 in the following: 
    import tvm
    import pickle
    a = tvm.const(0.00000001, 'float32')
    a2 = pickle.loads(pickle.dumps(a))

* remove line end spaces

src/node/serialization.cc

@@ -167,7 +167,11 @@ class JSONAttrGetter : public AttrVisitor {
   ReflectionVTable* reflection_ = ReflectionVTable::Global();
 
   void Visit(const char* key, double* value) final {
-    node_->attrs[key] = std::to_string(*value);
+    std::ostringstream s;
+    // Type <double> have approximately 16 decimal digits
+    s.precision(16);
+    s << (*value);
+    node_->attrs[key] = s.str();
   }
   void Visit(const char* key, int64_t* value) final {
     node_->attrs[key] = std::to_string(*value);