fix incorrect error message in convolution_expand_param_if_needed (#6409)

diff --git a/aten/src/ATen/native/Convolution.cpp b/aten/src/ATen/native/Convolution.cpp
index dba3988..9a1f13b 100644
--- a/aten/src/ATen/native/Convolution.cpp
+++ b/aten/src/ATen/native/Convolution.cpp
@@ -287,7 +287,7 @@
     std::ostringstream ss;
     ss << "expected " << param_name << " to be a single integer value or a "
        << "list of " << expected_dim << " values to match the convolution "
-       << "dimensions, but got " << param_name << "=" << list_param.size();
+       << "dimensions, but got " << param_name << "=" << list_param;
     throw std::runtime_error(ss.str());
   } else {
     return list_param.vec();