D2Go Fail Fast: Move exception coming from not implemented "compare accuracy" feature to the top.

Summary:
Currently, the exporter method takes in a compare_accuracy parameter, which after all the compute (exporting etc.) raises an exception if it is set to True.

This looks like an antipattern, and causes a waste of compute.

Therefore, I am proposing to raise the exception at the very beginning of method call to let the client know in advance that this argument's functionality isn't implemented yet.

NOTE: We might also choose to get rid of the entire parameter. I am open for suggestions.

Differential Revision: D34186578

fbshipit-source-id: d7fbe7589dfe2d2f688b870885ca61e6829c9329

tools/exporter.py

@@ -34,6 +34,13 @@ def main(
     compare_accuracy: bool = False,
     skip_if_fail: bool = False,
 ):
+    if compare_accuracy:
+        raise NotImplementedError(
+            "compare_accuracy functionality isn't currently supported."
+        )
+        # NOTE: dict for metrics of all exported models (and original pytorch model)
+        # ret["accuracy_comparison"] = accuracy_comparison
+
     cfg = copy.deepcopy(cfg)
     setup_after_launch(cfg, output_dir, runner)
 
@@ -72,10 +79,6 @@ def main(
                 raise e
 
     ret = {"predictor_paths": predictor_paths, "accuracy_comparison": {}}
-    if compare_accuracy:
-        raise NotImplementedError()
-        # NOTE: dict for metrics of all exported models (and original pytorch model)
-        # ret["accuracy_comparison"] = accuracy_comparison
 
     return ret