Fix Base Model Name of LlamaForQuestionAnswering (#29258)

* LlamaForQuestionAnswering self.transformer->self.model

* fix "Copied from" string

* Llama QA model: set base_model_prefix = "transformer"

src/transformers/models/llama/modeling_llama.py

@@ -1454,6 +1454,8 @@ SQuAD (a linear layer on top of the hidden-states output to compute `span start
     LLAMA_START_DOCSTRING,
 )
 class LlamaForQuestionAnswering(LlamaPreTrainedModel):
+    base_model_prefix = "transformer"
+
     # Copied from transformers.models.bloom.modeling_bloom.BloomForQuestionAnswering.__init__ with Bloom->Llama
     def __init__(self, config):
         super().__init__(config)