Fix logic bug in checking ffcv

Fixes #5.  Now we check for ffcv after we have properly determined if
we need it (which is in the tsimcne.fit() function call).

tsimcne/tsimcne.py

@@ -341,7 +341,9 @@ def _handle_parameters(self):
                 f"but got {self.freeze_schedule}."
             )
 
-        if self.use_ffcv:
+    @staticmethod
+    def check_ffcv(use_ffcv):
+        if use_ffcv:
             try:
                 import ffcv
 
@@ -406,6 +408,7 @@ def fit(self, X: torch.utils.data.Dataset | str):
                 self.use_ffcv = True
             else:
                 self.use_ffcv = False
+        self.check_ffcv(self.use_ffcv)
 
         train_dl = self.make_dataloader(X, True, None)