[MINOR] [ML] override toString of AttributeGroup

This makes equality test failures much more readable.

mengxr

Author: Eric Liang <ekl@databricks.com>
Author: Eric Liang <ekhliang@gmail.com>

Closes #8826 from ericl/attrgroupstr.

mllib/src/main/scala/org/apache/spark/ml/attribute/AttributeGroup.scala

@@ -183,6 +183,8 @@ class AttributeGroup private (
     sum = 37 * sum + attributes.map(_.toSeq).hashCode
     sum
   }
+
+  override def toString: String = toMetadata.toString
 }
 
 /**