text: Title: ENH: lazy init pandas fallbacks
  Body: ### Is your feature request related to a problem? Please describe
  To reduce the import time, we can init pandas fallback methods when users actually need them.
  
  You can refer to the impl of [numpy fallbacks](https://github.com/xprobe-inc/xorbits/blob/0ad7c68d5517c2368653652ea14971a9dd2f6613/python/xorbits/numpy/random/__init__.py#L38).
  
  You should also init pandas fallbacks when `__dir__` being called.