[Bugfix] Add logs for all model dtype casting (vllm-project#4717)

vllm/config.py

@@ -1063,6 +1063,7 @@ def _get_and_verify_dtype(
             if config_dtype == torch.float32:
                 # Following the common practice, we use float16 for float32
                 # models.
+                logger.info("Casting torch.float32 to torch.float16.")
                 torch_dtype = torch.float16
             else:
                 torch_dtype = config_dtype
@@ -1087,9 +1088,11 @@ def _get_and_verify_dtype(
     if torch_dtype != config_dtype:
         if torch_dtype == torch.float32:
             # Upcasting to float32 is allowed.
+            logger.info("Upcasting %s to %s.", config_dtype, torch_dtype)
             pass
         elif config_dtype == torch.float32:
             # Downcasting from float32 to float16 or bfloat16 is allowed.
+            logger.info("Downcasting %s to %s.", config_dtype, torch_dtype)
             pass
         else:
             # Casting between float16 and bfloat16 is allowed with a warning.