Add an option --fp16-unet to force using fp16 for the unet.

comfy/model_management.py

@@ -466,6 +466,8 @@ def unet_inital_load_device(parameters, dtype):
 def unet_dtype(device=None, model_params=0):
     if args.bf16_unet:
         return torch.bfloat16
+    if args.fp16_unet:
+        return torch.float16
     if args.fp8_e4m3fn_unet:
         return torch.float8_e4m3fn
     if args.fp8_e5m2_unet: