fix: dispatch LLMLingua-2 models by loosened name markers (#168)

[ousamabenyounes]

What does this PR do?

Fixes #168

`is_begin_of_new_word` and `get_pure_token` in `llmlingua/utils.py` dispatch on literal substrings — `"bert-base-multilingual-cased"` for the BERT family and `"xlm-roberta-large"` for the XLM-RoBERTa family. Users who download LLMLingua-2 weights and load them from a renamed local directory (for example the reporter's `/home/webservice/llm/compressFromNet/llmlingua-2-xlm`) fall straight through to `raise NotImplementedError` deep inside `compress_prompt_llmlingua2`, with no hint of what went wrong.

Fix

Factor the dispatch into `_model_family(model_name)` and broaden the marker lists:

Family	Old markers	New markers
BERT	`bert-base-multilingual-cased`, `tinybert`, `mobilebert`	+ `bert-base-multilingual` (no `-cased` suffix), `llmlingua-2-bert`
XLM-RoBERTa	`xlm-roberta-large`, `slingua`, `securitylingua`	+ `xlm-roberta` (no `-large` suffix), `llmlingua-2-xlm`

`None`/empty `model_name` now returns `None` cleanly instead of crashing on the old `"..." in model_name` check, and the fallback `NotImplementedError` now lists the supported markers so diagnosing a bad model_name no longer requires reading the source.

The canonical model names, `tinybert`, `mobilebert`, `slingua` and `securitylingua` all keep working unchanged.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Was this discussed/approved via a Github issue? Please add a link to it if that's the case — [Bug]: NotImplementedError #168.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? — `tests/test_issue_168.py` has 17 new tests (runs in ~5s, no model download): the reporter's exact local path, canonical XLM-RoBERTa / BERT names, bare `xlm-roberta-base`, `tinybert`, `mobilebert`, `slingua`, unknown models, empty/`None` input, and end-to-end `is_begin_of_new_word` / `get_pure_token` round-trips for both local-path families.

Verification

• Baseline: 2 tests pass, 3 tests fail with a pre-existing `ValueError: too many values to unpack (expected 2)` in `iterative_compress_prompt` (transformers DynamicCache API change — addressed separately in fix: normalize past_key_values across transformers DynamicCache API (#210) #246).
• Post-fix: 2 baseline tests still pass + 17 new tests pass (19 total), same 3 pre-existing failures — zero new regressions.

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