fix(mge): recover bn freeze fastpath execution

GitOrigin-RevId: 13e58c9fba9638ddb1820f4629488086185a8337

imperative/python/megengine/module/batchnorm.py

@@ -100,6 +100,15 @@ class _BatchNorm(Module):
             if _bias is not None:
                 _bias = _bias.detach()
 
+            # fastpath excution for freeze
+            scale = (self.running_var + self.eps) ** (-0.5)
+            if _weight is not None:
+                scale *= _weight
+            bias = -self.running_mean * scale
+            if _bias is not None:
+                bias += _bias
+            return inp * scale + bias
+
         if self.training and self.track_running_stats:
             exponential_average_factor = self.momentum
         else: