CTranslate2 1.8.0

New features

[Python] Add methods Translator.unload_model and Translator.load_model to manually manage memory
[Docker] Move all images to Python 3 only
Expose options that enable an internal sorting by length to increase the translation efficiency:
- for file translation: read_batch_size contiguous examples will be loaded, sorted by length, and batched with size max_batch_size
- for batch translation: if the batch is larger than max_batch_size, examples will be sorted by length and batched with size max_batch_size

Fix another error when releasing a translator that is placed on a GPU that is not GPU 0
Fix possible memory corruption when creating GPU translators in parallel
Fix memory that is briefly allocated on GPU 0 when destroying a translator that is placed on another GPU
Reduce latency of model loading, especially on GPU