Update implementation of PDARTS export due to mutator refactor (#2423)

* Disable precondition check for length in export for PDARTS

* update

src/sdk/pynni/nni/nas/pytorch/pdarts/mutator.py

@@ -55,7 +55,8 @@ def __init__(self, model, pdarts_epoch_index, pdarts_num_to_drop, switches={}):
                             del module[index]
                 assert len(module) <= len(choices), "Failed to remove dropped choices."
 
-    def sample_final(self):
+    def export(self):
+        # Cannot rely on super().export() because P-DARTS has deleted some of the choices and has misaligned length.
         results = super().sample_final()
         for mutable in self.mutables:
             if isinstance(mutable, LayerChoice):