community[minor]: Bedrock async methods (#12477)

Description: Added support for asynchronous streaming in the Bedrock
class and corresponding tests.

Primarily:
  async def aprepare_output_stream
    async def _aprepare_input_and_invoke_stream
    async def _astream
    async def _acall

I've ensured that the code adheres to the project's linting and
formatting standards by running make format, make lint, and make test.

Issue: #12054, #11589

Dependencies: None

Tag maintainer: @baskaryan 

Twitter handle: @dominic_lovric

---------

Co-authored-by: Piyush Jain <piyushjain@duck.com>

libs/community/tests/unit_tests/llms/test_bedrock.py

@@ -1,6 +1,14 @@
+import json
+from typing import AsyncGenerator, Dict
+from unittest.mock import MagicMock, patch
+
 import pytest
 
-from langchain_community.llms.bedrock import ALTERNATION_ERROR, _human_assistant_format
+from langchain_community.llms.bedrock import (
+    ALTERNATION_ERROR,
+    Bedrock,
+    _human_assistant_format,
+)
 
 TEST_CASES = {
     """Hey""": """
@@ -250,3 +258,51 @@ def test__human_assistant_format() -> None:
         else:
             output = _human_assistant_format(input_text)
             assert output == expected_output
+
+
+# Sample mock streaming response data
+MOCK_STREAMING_RESPONSE = [
+    {"chunk": {"bytes": b'{"text": "nice"}'}},
+    {"chunk": {"bytes": b'{"text": " to meet"}'}},
+    {"chunk": {"bytes": b'{"text": " you"}'}},
+]
+
+
+async def async_gen_mock_streaming_response() -> AsyncGenerator[Dict, None]:
+    for item in MOCK_STREAMING_RESPONSE:
+        yield item
+
+
+@pytest.mark.asyncio
+async def test_bedrock_async_streaming_call() -> None:
+    # Mock boto3 import
+    mock_boto3 = MagicMock()
+    mock_boto3.Session.return_value.client.return_value = (
+        MagicMock()
+    )  # Mocking the client method of the Session object
+
+    with patch.dict(
+        "sys.modules", {"boto3": mock_boto3}
+    ):  # Mocking boto3 at the top level using patch.dict
+        # Mock the `Bedrock` class's method that invokes the model
+        mock_invoke_method = MagicMock(return_value=async_gen_mock_streaming_response())
+        with patch.object(
+            Bedrock, "_aprepare_input_and_invoke_stream", mock_invoke_method
+        ):
+            # Instantiate the Bedrock LLM
+            llm = Bedrock(
+                client=None,
+                model_id="anthropic.claude-v2",
+                streaming=True,
+            )
+            # Call the _astream method
+            chunks = [
+                json.loads(chunk["chunk"]["bytes"])["text"]  # type: ignore
+                async for chunk in llm._astream("Hey, how are you?")
+            ]
+
+    # Assertions
+    assert len(chunks) == 3
+    assert chunks[0] == "nice"
+    assert chunks[1] == " to meet"
+    assert chunks[2] == " you"