[BB3] prefix space

[klshuster]

Patch description
Option for adding a prefix space after final prefix

Testing steps
pytest tests/nightly/gpu/test_bb3.py

(parlai_py39_pyt113) kshuster@devfair0140:~/ParlAI/tests$ pytest nightly/gpu/test_bb3.py
=============================================================================== test session starts ===============================================================================
platform linux -- Python 3.9.15, pytest-5.3.2, py-1.11.0, pluggy-0.13.1
rootdir: /private/home/kshuster/ParlAI, inifile: pytest.ini
plugins: requests-mock-1.7.0, hydra-core-1.2.0, datadir-1.4.1, regressions-2.1.1
collected 28 items

nightly/gpu/test_bb3.py .................s..........                                                                                                                        [100%]

============================================================================ slowest 10 test durations ============================================================================
45.01s call     tests/nightly/gpu/test_bb3.py::TestOptDecisionCombos::test_decision_combos
1.55s call     tests/nightly/gpu/test_bb3.py::TestIgnoreInSessionMemories::test_in_session_memories
1.25s call     tests/nightly/gpu/test_bb3.py::TestMemoryTFIDF::test_memory_tfidf
1.10s call     tests/nightly/gpu/test_bb3.py::TestMemorySoftBlockThreshold::test_softblocking
0.80s call     tests/nightly/gpu/test_bb3.py::TestOptBatching::test_batching
0.78s call     tests/nightly/gpu/test_bb3.py::TestMemoryNTurnsHardBlocking::test_hard_blocking
0.77s call     tests/nightly/gpu/test_bb3.py::TestOptOpening::test_opening
0.75s call     tests/nightly/gpu/test_bb3.py::TestMemoryOverlapThresholdBlocking::test_overlap_threshold_blocking
0.74s call     tests/nightly/gpu/test_bb3.py::TestOptOpening::test_failed_opening
0.74s call     tests/nightly/gpu/test_bb3.py::TestInjectQueryString::test_inject_query_string
=================================================================== 27 passed, 1 skipped, 15 warnings in 58.02s ===================================================================