message: Add logging in case we fallback to the secondary backend.
  This makes it possible to see from the logs whether the failover compiler used
  the secondary (classic xla gpu) backend instead of the mlir_gpu backend.
diff: @@ -32,6 +32,8 @@ StatusOr<std::unique_ptr<HloModule>> FailoverCompiler::RunHloPasses(
  auto result =
  primary_->RunHloPasses(module->Clone(), stream_exec, device_allocator);
  if (IsUnimplemented(result)) {
  + VLOG(2) << "RunHloPasses resulted in " << result.status()
  + << ", falling back to secondary backend";
  return secondary_->RunHloPasses(std::move(module), stream_exec,
  device_allocator);
  }
  @@ -44,6 +46,8 @@ StatusOr<std::unique_ptr<Executable>> FailoverCompiler::RunBackend(
  auto result =
  primary_->RunBackend(module->Clone(), stream_exec, device_allocator);
  if (IsUnimplemented(result)) {
  + VLOG(2) << "RunBackend resulted in " << result.status()
  + << ", falling back to secondary backend";
  return secondary_->RunBackend(std::move(module), stream_exec,
  device_allocator);
  }
  @@ -78,6 +82,8 @@ StatusOr<std::vector<std::unique_ptr<Executable>>> FailoverCompiler::Compile(
  }(modules[i]->Clone());
  if (IsUnimplemented(executable)) {
  + VLOG(2) << "Compile resulted in " << executable.status()
  + << ", falling back to secondary backend";
  TF_ASSIGN_OR_RETURN(
  modules[i],
  secondary_->RunHloPasses(std::move(modules[i]), stream_execs[i][0],
  