Longform QA demo breaks after clearing cache
## Environment info

Browser: Chrome, running on windows.

Running demo at: http://35.226.96.115:8080/

Linked from https://github.com/huggingface/transformers/tree/master/examples/longform-qa

### Who can help
@sgugger
 
## Information
I had clicked "Clear Cache" in the app and when I did another search errors came up in the browers. subsequent runs also produces errors in the browser.

RuntimeError: Error in void faiss::gpu::allocMemorySpaceV(faiss::gpu::MemorySpace, void**, size_t) at gpu/utils/MemorySpace.cpp:26: Error: 'err == cudaSuccess' failed: failed to cudaMalloc 8987501056 bytes (error 2 out of memory)
Traceback:
File "/home/yacine/anaconda3/lib/python3.7/site-packages/streamlit/ScriptRunner.py", line 322, in _run_script
    exec(code, module.__dict__)
File "/home/yacine/Code/transformers/examples/longform-qa/eli5_app.py", line 78, in <module>
    passages, gpu_dense_index, es_client = load_indexes()
File "/home/yacine/anaconda3/lib/python3.7/site-packages/streamlit/caching.py", line 591, in wrapped_func
    return get_or_create_cached_value()
File "/home/yacine/anaconda3/lib/python3.7/site-packages/streamlit/caching.py", line 575, in get_or_create_cached_value
    return_value = func(*args, **kwargs)
File "/home/yacine/Code/transformers/examples/longform-qa/eli5_app.py", line 58, in load_indexes
    wiki40b_gpu_index_flat.add(wiki40b_passage_reps)  # TODO fix for larger GPU
File "/home/yacine/anaconda3/lib/python3.7/site-packages/faiss/__init__.py", line 138, in replacement_add
    self.add_c(n, swig_ptr(x))
File "/home/yacine/anaconda3/lib/python3.7/site-packages/faiss/swigfaiss.py", line 4245, in add
    return _swigfaiss.GpuIndexFlat_add(self, arg2, x)
## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->

Expected to have the results of the ELI5 search
This seems to be an out-of-memory error! @yjernite might know what's up.<|||||>This issue has been automatically marked as stale and been closed because it has not had recent activity. Thank you for your contributions.

If you think this still needs to be addressed please comment on this thread.