message: Add a method to retrieve the shared_ptr for the PjRtBuffer.
diff: @@ -70,6 +70,7 @@ class PyBuffer {
  std::shared_ptr<PyClient> client() const { return client_; }
  PjRtBuffer* buffer() const { return buffer_.get(); }
  + std::shared_ptr<PjRtBuffer> shared_ptr_buffer() const { return buffer_; }
  ClientAndPtr<PjRtDevice> device() const;
  absl::string_view platform_name() const {
  