Add ShardRef type and implement ParamVar and ShardsVar classes

- Introduced a new VarType case for ShardRef to represent references to shards.
- Implemented ParamVar class for managing parameters with cleanup and warmup functionality.
- Developed ShardsVar class to handle collections of shard references, including methods for setting parameters, composing shards, and managing lifecycle.
- Removed obsolete ShardParameter and ShardController classes to streamline the codebase.

Author: sinkingsugar