update instructor for sentence transformers

[BBC-Esq]

Necessary because sentence transformers is now using the "backend" parameter when instantiating a model and the passing of "module_kwargs = {}" due to recent changes with sentence transformers.

This differs from the following two other pull requests that attempt to address the same issue, as outlined below...but the core unique feature of mine is that it's backward compatible with older versions of the sentence-transformers library:

Aspect	My Approach	PR #128	PR #127
Backend Parameter Implementation	Uses inspect.signature() to check if backend parameter exists at runtime	Uses Literal type for strict type checking: backend: Literal['torch', 'onnx', 'openvino'] = 'torch'	Uses simple string parameter: backend: str = "torch"
Parameter Passing	Adds backend to model_args if detected	Passes backend directly in constructor signature	Passes backend as separate positional argument to _load_model
Type Safety	Medium - Runtime checking but no static type hints	High - Uses Python type hints to enforce valid values	Low - Simple string type without validation
Backward Compatibility	High - Only uses backend parameter if supported	Low - Requires backend parameter	Medium - Always requires backend but without strict validation