community: refactor Baseten integration with new API endpoints & docs (#15017)

- **Description:** In response to user feedback, this PR refactors the
Baseten integration with updated model endpoints, as well as updates
relevant documentation. This PR has been tested by end users in
production and works as expected.
  - **Issue:** N/A
- **Dependencies:** This PR actually removes the dependency on the
`baseten` package!
  - **Twitter handle:** https://twitter.com/basetenco

libs/community/tests/integration_tests/llms/test_baseten.py

@@ -3,12 +3,11 @@ import os
 
 from langchain_community.llms.baseten import Baseten
 
+# This test requires valid BASETEN_MODEL_ID and BASETEN_API_KEY environment variables
+
 
 def test_baseten_call() -> None:
     """Test valid call to Baseten."""
-    import baseten
-
-    baseten.login(os.environ["BASETEN_API_KEY"])
     llm = Baseten(model=os.environ["BASETEN_MODEL_ID"])
-    output = llm("Say foo:")
+    output = llm("Test prompt, please respond.")
     assert isinstance(output, str)