message: Add a method to PyClient returning std::shared_ptr<PjRtClient>.
diff: @@ -91,6 +91,7 @@ class PyClient : public std::enable_shared_from_this<PyClient> {
  explicit PyClient(std::shared_ptr<PjRtClient> pjrt_client);
  PjRtClient* pjrt_client() const { return pjrt_client_.get(); }
  + std::shared_ptr<PjRtClient> shared_pjrt_client() { return pjrt_client_; }
  const std::string& platform_name() const {
  return pjrt_client_->platform_name();
  