API: Add default for backend in model load request

Should be None so pydantic doesn't complain.

Signed-off-by: kingbri <8082010+kingbri1@users.noreply.github.com>

endpoints/core/types/model.py

@@ -83,6 +83,7 @@ class ModelLoadRequest(BaseModel):
     # Config arguments
     backend: Optional[str] = Field(
         description="Backend to use",
+        default=None,
     )
     max_seq_len: Optional[int] = Field(
         description="Leave this blank to use the model's base sequence length",