fix transformers 5.x API breaks

[niradler]

Three API changes in transformers 5.x break qwen-asr with transformers >=5.0.

1. check_model_inputs removed

The decorator @check_model_inputs() was removed in transformers 5.x. The import raises ImportError at module load time, so qwen_asr cannot be imported at all with transformers 5.x.

Fix: guard the import with try/except and fall back to a transparent no-op decorator.

2. ROPE_INIT_FUNCTIONS default key removed

The default key was removed from ROPE_INIT_FUNCTIONS in transformers 5.x. Qwen3ASRThinkerTextRotaryEmbedding.init indexes into it with self.rope_type which defaults to default, raising KeyError at model load time.

Fix: fall back to a self-contained RoPE parameter function when the key is missing. Also adds _compute_default_rope_parameters as a method, since the @dynamic_rope_update decorator in transformers 5.x calls it.

3. pad_token_id missing on Qwen3ASRThinkerConfig

GenerationMixin in transformers 5.x requires pad_token_id to be explicitly present on the config object. The attribute is not declared as a class-level default on Qwen3ASRThinkerConfig, causing AttributeError during generation.

Fix: declare pad_token_id = None as a class-level default.