Add AI21 LLMs (#99)

Integrate AI21 /complete API into langchain, to allow access to Jurassic
models.

tests/integration_tests/llms/test_ai21.py

@@ -0,0 +1,10 @@
+"""Test AI21 API wrapper."""
+
+from langchain.llms.ai21 import AI21
+
+
+def test_ai21_call() -> None:
+    """Test valid call to ai21."""
+    llm = AI21(maxTokens=10)
+    output = llm("Say foo:")
+    assert isinstance(output, str)