The lightning network is not a fork nor does it do anything with Bitcoin's consensus rules. It can't really be hard forked unless the underlying asset (aka Bitcoin) is forked. LN works on top of the blockchain; it is not its own separate coin with its own consensus rules.
It's a part? Means it can be called sub-blockchain to let others transact off-chain?
But, as there are on-chain forks possible, there may come something like off-chain forks too if they are not satisfied with the delivered results and there may arise problems with it too (maybe) where an upgrade may be expected and a "possibly soft if not hard" fork may be required?
Please correct me if I am wrong with my understanding about LN as mentioned below:
LN is something that allows transactions at almost 0 cost to people who create a channel and do transactions in their created network, and everything gets recorded off-chain and there's no need for validation of these transactions? How will they validated unless they go on-chain? Is double-spending a possibility?