message: Improve error message on broadcast error
  Summary: Just improve error message
  Test Plan: None
  Reviewers: georgep
diff: @@ -28,6 +28,7 @@ limitations under the License.
  #include "tensorflow/compiler/xla/service/hlo_instruction.h"
  #include "tensorflow/compiler/xla/shape_util.h"
  #include "tensorflow/compiler/xla/status_macros.h"
  +#include "tensorflow/core/lib/strings/str_util.h"
  #include "tensorflow/core/lib/strings/strcat.h"
  #include "tensorflow/core/util/bcast.h"
  #include "tensorflow/stream_executor/lib/status.h"
  @@ -36,6 +37,7 @@ limitations under the License.
  #include <poplar/OptionFlags.hpp>
  #include <poputil/TileMapping.hpp>
  +using ::tensorflow::str_util::Join;
  using ::tensorflow::strings::StrCat;
  namespace xla {
  @@ -652,7 +654,9 @@ StatusOr<poplar::Tensor> BroadcastTensor(const poplar::Tensor& in,
  tensorflow::BCast bcast(tensor_shape, bcast_shape);
  if (!bcast.IsValid()) {
  - return xla::FailedPrecondition("Incompatible broadcast");
  + return xla::FailedPrecondition("Incompatible broadcast from (",
  + Join(tensor_shape, ","), ") to (",
  + Join(bcast_shape, ","), ")");
  }
  poplar::Tensor o = in;
  