fix: Raise a clear error when HFSystemFormat's tokenizer has no chat template

[wak327]

Summary

HFSystemFormat renders prompts with tokenizer.apply_chat_template. When the model's tokenizer has no chat_template (e.g. ibm-granite/granite-20b-code-base from #1144, or gpt2), every instance fails while the dataset is being generated with transformers' generic error:

ValueError: Cannot use chat template functions because tokenizer.chat_template is not set and no template argument was passed! ...

That message doesn't name the model, and it doesn't mention the options unitxt already supports.

Changes

• HFSystemFormat.prepare() checks the tokenizer right after loading it. If there is no chat template and none was passed through chat_kwargs_dict, it raises a UnitxtError once, when the format is created:

  HFSystemFormat cannot be used with model 'openai-community/gpt2' because its tokenizer does not define a chat template (no 'chat_template' in its tokenizer_config.json). Either use a model whose tokenizer has a chat template, pass a Jinja chat template explicitly with chat_kwargs_dict={'chat_template': '<template>'}, or use a format that does not depend on the tokenizer, such as SystemFormat.
  

• The HFSystemFormat docstring now mentions passing a template through chat_kwargs_dict={"chat_template": ...}. That already works today, since chat_kwargs_dict is forwarded to apply_chat_template.

• UnitxtError isn't in the retry list of retry_connection_with_exponential_backoff, so the new check is not retried.

Fixes #1144

Test plan

• New test_hf_system_format_without_chat_template: creating the format with openai-community/gpt2 raises the UnitxtError, and passing an explicit chat_template in chat_kwargs_dict formats correctly. Fails on main (UnitxtError not raised), passes here.
• Existing test_hf_system_format (zephyr) still passes.
• pre-commit run on the changed files.

[wak327]

@elronbandel @yoavkatz could you take a look when you have a moment? Small change: HFSystemFormat now fails early with a clear message when the model's tokenizer has no chat template (#1144), instead of failing on every instance with the generic transformers error.