fix: Add old parameter for backwards compatibility

Signed-off-by: Will Johnson <[email protected]>

fixtures/accelerate_fsdp_defaults.yaml

@@ -17,6 +17,7 @@ fsdp_config:
   fsdp_backward_prefetch: BACKWARD_PRE # set to BACKWARD_PRE for the most time-efficient pipeline
                                               # but requires the most memory. BACKWARD_POST is the less
                                               # memory intensive option
+  fsdp_backward_prefetch_policy: BACKWARD_PRE # for backwards compatibility
 
   # setting this to true will increase forward memory by prefetching the next FSDP all-gather, while performing
   # the current forward pass.