Assert on empty PJRT buffers

[rpsilva-aws]

Simple CR to avoid a segmentation fault when there are placeholder tensors involved, as we attempt to de-reference the device from the buffer. It fixes the seg fault for #9049.

ERROR:root:Caught an exception when exiting the process. Exception: 
Traceback (most recent call last):
  File "/ansible/pytorch/xla/torch_xla/__init__.py", line 209, in _prepare_to_exit
    _XLAC._prepare_to_exit()
RuntimeError: torch_xla/csrc/runtime/pjrt_computation_client.cc:741 : Check failed: pjrt_data->buffer != nullptr 
*** Begin stack trace ***
        tsl::CurrentStackTrace[abi:cxx11]()
        torch_xla::runtime::PjRtComputationClient::ExecuteComputation(torch_xla::runtime::ComputationClient::Computation const&, absl::lts_20230802::Span<std::shared_ptr<torch_xla::runtime::ComputationClient::Data> const>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, torch_xla::runtime::ComputationClient::ExecuteComputationOptions const&)




        torch::lazy::MultiWait::Complete(std::function<void ()> const&)
        std::function<void ()>::operator()() const
        tsl::thread::EigenEnvironment::ExecuteTask(tsl::thread::EigenEnvironment::Task const&)
        Eigen::ThreadPoolTempl<tsl::thread::EigenEnvironment>::WorkerLoop(int)
        Eigen::ThreadPoolTempl<tsl::thread::EigenEnvironment>::ThreadPoolTempl(int, bool, tsl::thread::EigenEnvironment)::{lambda()#1}::operator()() const
        void std::__invoke_impl<void, Eigen::ThreadPoolTempl<tsl::thread::EigenEnvironment>::ThreadPoolTempl(int, bool, tsl::thread::EigenEnvironment)::{lambda()#1}&>(std::__invoke_other, Eigen::ThreadPoolTempl<tsl::thread::EigenEnvironment>::ThreadPoolTempl(int, bool, tsl::thread::EigenEnvironment)::{lambda()#1}&)
        std::enable_if<is_invocable_r_v<void, Eigen::ThreadPoolTempl<tsl::thread::EigenEnvironment>::ThreadPoolTempl(int, bool, tsl::thread::EigenEnvironment)::{lambda()#1}&>, std::enable_if>::type std::__invoke_r<void, Eigen::ThreadPoolTempl<tsl::thread::EigenEnvironment>::ThreadPoolTempl(int, bool, tsl::thread::EigenEnvironment)::{lambda()#1}&>(void&&, (Eigen::ThreadPoolTempl<tsl::thread::EigenEnvironment>::ThreadPoolTempl(int, bool, tsl::thread::EigenEnvironment)::{lambda()#1}&)...)
        std::_Function_handler<void (), Eigen::ThreadPoolTempl<tsl::thread::EigenEnvironment>::ThreadPoolTempl(int, bool, tsl::thread::EigenEnvironment)::{lambda()#1}>::_M_invoke(std::_Any_data const&)
        std::function<void ()>::operator()() const
        tsl::thread::EigenEnvironment::CreateThread(std::function<void ()>)::{lambda()#1}::operator()() const
        void std::__invoke_impl<void, tsl::thread::EigenEnvironment::CreateThread(std::function<void ()>)::{lambda()#1}&>(std::__invoke_other, tsl::thread::EigenEnvironment::CreateThread(std::function<void ()>)::{lambda()#1}&)
        std::__invoke_result<tsl::thread::EigenEnvironment::CreateThread(std::function<void ()>)::{lambda()#1}&>::type std::__invoke<tsl::thread::EigenEnvironment::CreateThread(std::function<void ()>)::{lambda()#1}&>(std::__invoke_result&&, (tsl::thread::EigenEnvironment::CreateThread(std::function<void ()>)::{lambda()#1}&)...)
        std::invoke_result<tsl::thread::EigenEnvironment::CreateThread(std::function<void ()>)::{lambda()#1}&>::type std::invoke<tsl::thread::EigenEnvironment::CreateThread(std::function<void ()>)::{lambda()#1}&>(std::invoke_result&&, (tsl::thread::EigenEnvironment::CreateThread(std::function<void ()>)::{lambda()#1}&)...)
        void absl::lts_20230802::internal_any_invocable::InvokeR<void, tsl::thread::EigenEnvironment::CreateThread(std::function<void ()>)::{lambda()#1}&, , void>(tsl::thread::EigenEnvironment::CreateThread(std::function<void ()>)::{lambda()#1}&)
        void absl::lts_20230802::internal_any_invocable::RemoteInvoker<false, void, tsl::thread::EigenEnvironment::CreateThread(std::function<void ()>)::{lambda()#1}&>(absl::lts_20230802::internal_any_invocable::TypeErasedState*)
        absl::lts_20230802::internal_any_invocable::Impl<void ()>::operator()()

[bhavya01]

Is it possible to add a test for this?

Looking at #9049 quickly, I didn't follow the path how calling _get_xla_tensors_hlo leads to ExecuteComputation call on the PjRtClient