[Performance] Move validation calls out of Resource.__init__

[yika-luo]

We are initiating the resource object multiple times which triggers non-trivial validation. https://github.com/skypilot-org/skypilot/blob/master/sky/resources.py#L228-L234

Version & Commit info:

• sky -v: PLEASE_FILL_IN
• sky -c: PLEASE_FILL_IN