feat(router.py): Support Loadbalancing batch azure api endpoints (#5469)

* feat(router.py): initial commit for loadbalancing azure batch api endpoints

Closes https://github.com/BerriAI/litellm/issues/5396

* fix(router.py): working `router.acreate_file()`

* feat(router.py): working router.acreate_batch endpoint

* feat(router.py): expose router.aretrieve_batch function

Make it easy for user to retrieve the batch information

* feat(router.py): support 'router.alist_batches' endpoint

Adds support for getting all batches across all endpoints

* feat(router.py): working loadbalancing on `/v1/files`

* feat(proxy_server.py): working loadbalancing on `/v1/batches`

* feat(proxy_server.py): working loadbalancing on Retrieve + List batch

litellm/router_utils/batch_utils.py

@@ -0,0 +1,59 @@
+import io
+import json
+from typing import IO, Optional, Tuple, Union
+
+
+class InMemoryFile(io.BytesIO):
+    def __init__(self, content: bytes, name: str):
+        super().__init__(content)
+        self.name = name
+
+
+def replace_model_in_jsonl(
+    file_content: Union[bytes, IO, Tuple[str, bytes, str]], new_model_name: str
+) -> Optional[InMemoryFile]:
+    try:
+        # Decode the bytes to a string and split into lines
+        # If file_content is a file-like object, read the bytes
+        if hasattr(file_content, "read"):
+            file_content_bytes = file_content.read()  # type: ignore
+        elif isinstance(file_content, tuple):
+            file_content_bytes = file_content[1]
+        else:
+            file_content_bytes = file_content
+
+        # Decode the bytes to a string and split into lines
+        file_content_str = file_content_bytes.decode("utf-8")
+        lines = file_content_str.splitlines()
+        modified_lines = []
+        for line in lines:
+            # Parse each line as a JSON object
+            json_object = json.loads(line.strip())
+
+            # Replace the model name if it exists
+            if "body" in json_object:
+                json_object["body"]["model"] = new_model_name
+
+            # Convert the modified JSON object back to a string
+            modified_lines.append(json.dumps(json_object))
+
+        # Reassemble the modified lines and return as bytes
+        modified_file_content = "\n".join(modified_lines).encode("utf-8")
+        return InMemoryFile(modified_file_content, name="modified_file.jsonl")  # type: ignore
+
+    except (json.JSONDecodeError, UnicodeDecodeError, TypeError) as e:
+        return None
+
+
+def _get_router_metadata_variable_name(function_name) -> str:
+    """
+    Helper to return what the "metadata" field should be called in the request data
+
+    For all /thread or /assistant endpoints we need to call this "litellm_metadata"
+
+    For ALL other endpoints we call this "metadata
+    """
+    if "batch" in function_name:
+        return "litellm_metadata"
+    else:
+        return "metadata"