community: Add Baichuan Embeddings batch size (#22942)

- **Support batch size** 
Baichuan updates the document, indicating that up to 16 documents can be
imported at a time

- **Standardized model init arg names**
    - baichuan_api_key -> api_key
    - model_name  -> model

libs/community/tests/unit_tests/embeddings/test_baichuan.py

@@ -0,0 +1,18 @@
+from typing import cast
+
+from langchain_core.pydantic_v1 import SecretStr
+
+from langchain_community.embeddings import BaichuanTextEmbeddings
+
+
+def test_sparkllm_initialization_by_alias() -> None:
+    # Effective initialization
+    embeddings = BaichuanTextEmbeddings(  # type: ignore[call-arg]
+        model="embedding_model",  # type: ignore[arg-type]
+        api_key="your-api-key",  # type: ignore[arg-type]
+    )
+    assert embeddings.model_name == "embedding_model"
+    assert (
+        cast(SecretStr, embeddings.baichuan_api_key).get_secret_value()
+        == "your-api-key"
+    )