PaddleOCR threadpooling

[jw098]

Deadlock occurs under certain situations.
To reproduce. Run the CommandLineTests, where TEST_LIST contains "PokemonLZA/FlavorPowerScreenDetector".
Deadlock seems to trigger when PaddleOCR and Tesseract are both called by the same function. e.g. FlavorPowerDetector::detect_power, since text OCR uses Paddle and number OCR uses Tesseract.

The deadlock is "fixed" if we switch number OCR to using PaddleOCR as well.

EDIT: This should be fixed now.

[Mysticial]

You don't need shared_ptr here since this owns the instance itself. Just stick with:
std::map<LanguageGroup, ML::PaddleOCRPipeline> ocr_pool;

Then when you construct it later on:

iter = ocr_pool.emplace(
    std::piecewise_construct,
    std::forward_as_tuple(language_group),
    std::forward_as_tuple(language)
).first;

Then you can ensure_paddle_ocr_instance() can just return ML::PaddleOCRPipeline&.