refactor(utils): lazy load CustomStreamWrapper to improve import time

- Move CustomStreamWrapper from streaming_handler to lazy loading via __getattr__
- Add type stub in TYPE_CHECKING block for mypy type checking
- CustomStreamWrapper is not used internally in utils.py, only exported for other modules

This reduces import time by deferring the streaming_handler module import until CustomStreamWrapper is actually accessed.

litellm/utils.py

@@ -99,7 +99,6 @@ from litellm.litellm_core_utils.llm_response_utils.get_headers import (
     get_response_headers,
 )
 from litellm.litellm_core_utils.rules import Rules
-from litellm.litellm_core_utils.streaming_handler import CustomStreamWrapper
 from litellm.llms.base_llm.google_genai.transformation import (
     BaseGoogleGenAIGenerateContentConfig,
 )
@@ -340,6 +339,7 @@ if TYPE_CHECKING:
         LiteLLMLoggingObject,
         redact_message_input_output_from_logging,
     )
+    from litellm.litellm_core_utils.streaming_handler import CustomStreamWrapper
 
 from litellm.llms.base_llm.batches.transformation import BaseBatchesConfig
 from litellm.llms.base_llm.chat.transformation import BaseConfig
@@ -8905,4 +8905,14 @@ def __getattr__(name: str) -> Any:  # noqa: PLR0915
             _globals["redact_message_input_output_from_logging"] = _redact_message_input_output_from_logging
         return _globals["redact_message_input_output_from_logging"]
     
+    # Lazy load CustomStreamWrapper to avoid loading at module import time
+    if name == "CustomStreamWrapper":
+        # Check if already cached
+        if "CustomStreamWrapper" not in _globals:
+            from litellm.litellm_core_utils.streaming_handler import (
+                CustomStreamWrapper as _CustomStreamWrapper,
+            )
+            _globals["CustomStreamWrapper"] = _CustomStreamWrapper
+        return _globals["CustomStreamWrapper"]
+    
     raise AttributeError(f"module {__name__!r} has no attribute {name!r}")