community[patch]: fix model initialization bug for deepinfra (#25727)

### Description
adds an init method to ChatDeepInfra to set the model_name attribute
accordings to the argument
### Issue
currently, the model_name specified by the user during initialization of
the ChatDeepInfra class is never set. Therefore, it always chooses the
default model (meta-llama/Llama-2-70b-chat-hf, however probably since
this is deprecated it always uses meta-llama/Llama-3-70b-Instruct). We
stumbled across this issue and fixed it as proposed in this pull
request. Feel free to change the fix according to your coding guidelines
and style, this is just a proposal and we want to draw attention to this
problem.
### Dependencies
no additional dependencies required

Feel free to contact me or @timo282 and @finitearth if you have any
questions.

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>
Co-authored-by: Bagatur <22008038+baskaryan@users.noreply.github.com>

libs/community/tests/unit_tests/chat_models/test_deepinfra.py

@@ -0,0 +1,11 @@
+from langchain_community.chat_models import ChatDeepInfra
+
+
+def test_deepinfra_model_name_param() -> None:
+    llm = ChatDeepInfra(model_name="foo")  # type: ignore[call-arg]
+    assert llm.model_name == "foo"
+
+
+def test_deepinfra_model_param() -> None:
+    llm = ChatDeepInfra(model="foo")
+    assert llm.model_name == "foo"