Ensure that torch is not used unless it is available

[svlandeg]

Description

Bugfix. If you had transformers installed but not torch, you would get an ugly error upon importing spacy_llm:

Traceback (most recent call last):
  File "...\spacy-llm\spacy_llm\__init__.py", line 1, in <module>
    from . import cache  # noqa: F401
  File "...\spacy-llm\spacy_llm\cache.py", line 11, in <module>
    from .ty import PromptTemplateProvider, ShardingLLMTask
  File "...\spacy-llm\spacy_llm\ty.py", line 14, in <module>
    from .models import langchain
  File "...\spacy-llm\spacy_llm\models\__init__.py", line 1, in <module>
    from .hf import dolly_hf, openllama_hf, stablelm_hf
  File "...\spacy-llm\spacy_llm\models\hf\__init__.py", line 7, in <module>
    from .registry import huggingface_v1
  File "...\spacy-llm\spacy_llm\models\hf\registry.py", line 12, in <module>
    from .stablelm import StableLM
  File "...\spacy-llm\spacy_llm\models\hf\stablelm.py", line 11, in <module>
    class _StopOnTokens(transformers.StoppingCriteria):
  File "...\spacy-llm\spacy_llm\models\hf\stablelm.py", line 13, in _StopOnTokens
    self, input_ids: torch.LongTensor, scores: torch.FloatTensor, **kwargs
AttributeError: 'NoneType' object has no attribute 'LongTensor'

Types of change

bug fix

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran all tests in tests and usage_examples/tests, and all new and existing tests passed. This includes
  • all external tests (i. e. pytest ran with --external)
  • all tests requiring a GPU (i. e. pytest ran with --gpu)
• My changes don't require a change to the documentation, or if they do, I've added all required information.