Trim trailing default values in saved xmls.

For attributes which accept variable-length inputs. Currently used for `gainprm, biasprm, friction, solref, solimp` Example:

- Saved before: `biasprm="0 -3 -4 0 0 0 0 0 0 0"`.
- Saved after: `biasprm="0 -3 -4"`.

PiperOrigin-RevId: 607609550
Change-Id: I0501348280663867693175236872e2aabc703601

src/xml/xml_util.h

@@ -195,7 +195,7 @@ class mjXUtil {
   // write attribute- any type
   template<typename T>
   static void WriteAttr(tinyxml2::XMLElement* elem, std::string name, int n, const T* data,
-                        const T* def = 0);
+                        const T* def = 0, bool trim = false);
 
   // write vector<double> attribute, with and without default
   static void WriteVector(tinyxml2::XMLElement* elem, std::string name,