partners: Fixed the procedure of initializing pad_token_id (#29500)

- **Description:** Add to check pad_token_id and eos_token_id of model
config. It seems that this is the same bug as the HuggingFace TGI bug.
It's same bug as #29434
- **Issue:** #29431
- **Dependencies:** none
- **Twitter handle:** tell14

Example code is followings:
```python
from langchain_huggingface.llms import HuggingFacePipeline

hf = HuggingFacePipeline.from_model_id(
    model_id="meta-llama/Llama-3.2-3B-Instruct",
    task="text-generation",
    pipeline_kwargs={"max_new_tokens": 10},
)

from langchain_core.prompts import PromptTemplate

template = """Question: {question}

Answer: Let's think step by step."""
prompt = PromptTemplate.from_template(template)

chain = prompt | hf

question = "What is electroencephalography?"

print(chain.invoke({"question": question}))
```

libs/partners/huggingface/langchain_huggingface/llms/huggingface_pipeline.py

@@ -202,7 +202,16 @@ class HuggingFacePipeline(BaseLLM):
         model = model_cls.from_pretrained(model_id, **_model_kwargs)
 
         if tokenizer.pad_token is None:
-            tokenizer.pad_token_id = model.config.eos_token_id
+            if model.config.pad_token_id is not None:
+                tokenizer.pad_token_id = model.config.pad_token_id
+            elif model.config.eos_token_id is not None and isinstance(
+                model.config.eos_token_id, int
+            ):
+                tokenizer.pad_token_id = model.config.eos_token_id
+            elif tokenizer.eos_token_id is not None:
+                tokenizer.pad_token_id = tokenizer.eos_token_id
+            else:
+                tokenizer.add_special_tokens({"pad_token": "[PAD]"})
 
         if (
             (